Climate Change and...
Effects of Climate Change
- Climate Variability
- Climate Models
Effects of Climate Change
Balling, R.C., Jr., S.G. Wells (1990). Historic rainfall patterns and arroyo activity within the Zuni River drainage basin, New Mexico. Annals of the Association of American Geographers 80 (4): 603-617
ABSTRACT: Climate change, grazing practices, timbering activities, and erosional thresholds have been proposed to explain wide-spread accelerated arroyo erosion near the turn of the century in the southwestern United States. We analyze the morphology and potential causes of arroyo activity in the Zuni River drainage basin of New Mexico; our analyses illustrate the linkage between arroyos and changes that occurred through time in local precipitation patterns. Substantial archival and geological evidence confirms dominant downcutting in the intermediate and small-size arroyos within the basin for a 20- to 30-year period beginning near 1905. Historical climatic records reveal a long and severe drought from 1898-1904; this drought ended abruptly with three years dominated by unusually frequent high-intensity summer rainfall events. Daily weather records show the following two to three decades had a high number of intense summer storms, large rainfall totals, and few precipitation days. The results add support for the climate change explanation of periods dominated by arroyo incision and infilling in the southwestern U.S.
Hereford, R. (1984). Climate and ephemeral stream processes: Twentieth century geomorphology and alluvial stratigraphy of the Little Colorado River, Arizona. Geological Society of America Bulletin 95 (6): 654-668
ABSTRACT: During the first 40 years of the twentieth century, erosion was the dominant geomorphic process affecting the morphology of the Little Colorado River channel. The discharge regimen was one of frequent large floods and high annual discharge that created a wide sandy channel free of vegetation. In the 1940s and early 1950s, average annual precipitation declined, reducing annual discharge to about 57% of that of the preceding period as well as reducing the frequency of large floods. The channel adjusted to the new hydrologic regimen by reducing its width. Parts of the channel were frequently dry, and riparian vegetation, primarily nonnative salt cedar, became established on the higher channel surfaces. Precipitation and discharge thereafter increased and aggradation by overbank deposition was the primary geomorphic process, as indicated by accretion of 2 to 5 m of flood-plain alluvium between 1952 and 1978. Events of 1980, however, suggest that the flood plain has ceased to accrete, although climate has not fluctuated. The flood plain has probably reached a critical height above the channel, beyond which further accretion is unlikely under the existing discharge regimen. The recent history of the Little Colorado broadly suggests that flood-plain development was initiated by climatically induced hydrologic fluctuations. Flood-plain deposits in the stratigraphic column of such ephemeral streams may record repeated adjustments to altered hydrologic conditions.
Hereford, R. (2002). Valley-fill alluviation during the Little Ice Age (ca. A.D. 1400–1880), Paria River basin and southern Colorado Plateau, United States. Geological Society of America Bulletin 114 (12): 1550-1563
ABSTRACT: Valley-fill alluvium deposited from ca. A.D. 1400 to 1880 is widespread in tributaries of the Paria River and is largely coincident with the Little Ice Age epoch of global climate variability. Previous work showed that alluvium of this age is a mappable stratigraphic unit in many of the larger alluvial valleys of the southern Colorado Plateau. The alluvium is bounded by two disconformities resulting from prehistoric and historic arroyo cutting at ca. A.D. 1200–1400 and 1860–1910, respectively. The fill forms a terrace in the axial valleys of major through-flowing streams. This terrace and underlying deposits are continuous and interfinger with sediment in numerous small tributary valleys that head at the base of hillslopes of sparsely vegetated, weakly consolidated bedrock, suggesting that eroded bedrock was an important source of alluvium along with in-channel and other sources. Paleoclimatic and high-resolution paleoflood studies indicate that valley-fill alluviation occurred during a long-term decrease in the frequency of large, destructive floods. Aggradation of the valleys ended about A.D. 1880, if not two decades earlier, with the beginning of historic arroyo cutting. This shift from deposition to valley entrenchment near the close of the Little Ice Age generally coincided with the beginning of an episode of the largest floods in the preceding 400–500 yr, which was probably caused by an increased recurrence and intensity of flood-producing El Niño events beginning at ca. A.D. 1870.
G.A. Meyer, S. G. Wells, A. J. T. Jull (1995). Fire and alluvial chronology in Yellowstone National Park: Climatic and intrinsic controls on Holocene geomorphic processes. Geological Society of America Bulletin 107 (10): 1211-1230
ABSTRACT: We employed a systemwide approach, a large and robust set of radiocarbon ages, and modern process analogs to interpret the Holocene history of forest fire–related sedimentation and overall alluvial activity in northeastern Yellowstone National Park. Debris-flow and flood events following the 1988 fires provided facies models for interpreting the stratigraphic record of fire-related sedimentation within valley-side alluvial fans of Soda Butte Creek. Fire-related deposits make up approximately 30% of the late Holocene fan alluvium. Fifty14 C ages on fire-related events cluster within the intervals of 3300–2900, 2600–2400, 2200–1800, and 1400–800 yr B.P. and suggest earlier episodes of large fires and fan aggradation around 7500, 5500, and 4600–4000 yr B.P. A major pulse of fire-related debris-flow activity between 950 and 800 yr B.P. coincided with the height of the widely recognized Medieval Warm Period (ca. a.d. 1050–1200). Instrumental climate records over the last 100 yr in Yellowstone imply that the intensity and interannual variability of summer precipitation are greater during warmer periods, enhancing the potential for severe short-term drought, major forest fires, and storm-generated fan deposition.
Along lower Soda Butte Creek, fill-cut terrace treads were created by lateral migration of channels and accumulation of overbank sediments ca. 8000 yr B.P. (terrace level T1a), 7000–5600 (T1b), 3100–2600 (T2), 2000–1300 (T3), and post–800 yr B.P. (T4). These periods coincide with overbank sedimentation on Slough Creek and the Lamar River but alternate with intervals of fire-related fan deposition, implying a strong climatic control. Local paleoclimatic data suggest cooler, effectively wetter conditions during terrace tread formation. In warmer, drier intervals, reduced average runoff in axial streams results in meander-belt narrowing; concurrent channel incision may be caused by infrequent large floods. Greater resistance to downcutting, however, allowed fewer terraces to be formed along Slough Creek and the Lamar River. Alluvial systems in northeastern Yellowstone show a clear response to millennial-scale climatic cycles, wherein alluvial fans aggrade and prograde over flood plains in drier periods. Axial streams widen their flood plains and trim back the fans during wetter periods. “Small-scale” climatic fluctuations of the Holocene thus had substantial impact on postglacial landscapes in northeastern Yellowstone.
ABSTRACT: The Stanley River in western Tasmania, Australia, contains sub-fossil rainforest logs within the channel and floodplain. Of the more than 85 radiocarbon dates obtained, all but 3 date from 17 ka to the present and permit an interpretation of fluvial and related environmental changes over this period. Particular attention is focused on the interactive relationship between the river and its riparian rainforest. Following the Last Glacial Maximum, the Stanley River was a laterally active gravel-load system reworking most of its valley floor in the upstream reaches. With ameliorating conditions at the end of the Pleistocene, climate became less seasonal and flow regimes less energetic. Huon pines already present in the catchment, re-asserted themselves in the form of dense tree cover along the river banks and floodplains with basal floodplain deposition shifting from gravels to coarse sands and granules. By about 3.5 ka, a further change in climate reduced stream discharges substantially. As a result the channel reduced in size, transported finer sediment, became laterally stable, and the floodplain accreted with overbank deposits of sand and silt. Huon pines falling into the channel formed obstructions of woody debris, some surviving for 2 ka. These have reduced stream power and boundary shear stress, further contributing to channel stability. Generational sequences of Huon pines on the river banks, some extending back 1–2 ka, are additional evidence of this stability. Since the Pleistocene, changing climate and the re-establishment of dense riparian rainforest appear to have stabilised the river channels and floodplains of western Tasmania.
Baker, R.G., G. G. Fredlund, R. D. Mandel, E. A. BettisIII (2000). Holocene environments of the central Great Plains: multi-proxy evidence from alluvial sequences, southeastern Nebraska. Quaternary International 67 (1): 75-88
ABSTRACT: Pollen, plant macrofossils, phytoliths, carbon isotopes, and alluvial history from sediments exposed along the South Fork of the Big Nemaha River, southeastern Nebraska, USA, provide an integrated reconstruction of changes in Holocene vegetation, climate, and fluvial activity. From 9000 to 8500 uncalibrated14 C yr BP, climate became more arid and the floodplain and alluvial fans in the main valley aggraded rapidly, upland deciduous forest declined, and prairie attained its Holocene dominance. From 8500 to 5800 yr BP. upland forest elements disappeared, and even riparian trees were sparse under dry climatic conditions. Alluvial fans continued to aggrade but aggradation in the main valley was interrupted by a stable episode 7000 yr BP. From 5800 to 3100 yr BP, riparian forests returned to prominence, and droughts were intermittent. Alluviation was slower and punctuated by two major episodes of channel incision and terrace formation in the main valley. Aggradation on alluvial fans slowed and finally ceased near the end of this period. During a short dry interval from 3100 to 2700 yr BP riparian trees (except elm) disappeared, and prairie and weedy species became more abundant. This interval is represented by the organic Roberts Creek Member, and the alluvial setting was a slightly incised meandering channel belt. Habitats became similar to presettlement conditions during the last 2700 yr BP. Weedy taxa dominate modern samples, reflecting widespread disturbance. Alluvial fans and terrace surfaces were stable during the last 2500 years, but episodes of floodplain aggradation were punctuated by incision of the main channel.
ABSTRACT: An understanding of the response of a fluvial system to past climatic changes is useful for predicting its response to future shifts in temperature and precipitation. To determine the response of the Columbia River system to previous climatic conditions and transitions, a well-dated sequence of floodplain development in the Wells Reservoir region was compared with the paleoenvironmental history of the Columbia River Basin. Results of this comparison indicate that aggradation episodes, occurring approximately 9000-8000, 7000-6500, 4400-3900, and 2400-1800 yr B.P., coincided with climatic transitions that share certain characteristics. The inferred climates associated with aggradation had at least moderate rates of precipitation that occurred mainly in winter coupled with moderate winter temperatures. Such conditions would have resulted in the buildup of snowpacks and a high frequency of rain-on-snow events. The warming and precipitation increases predicted for the Pacific Northwest under most CO2 -doubling scenarios are likely to repeat these conditions, which could increase the frequency of severe, sediment-laden floods in the Columbia River Basin.
ABSTRACT: Field investigations at Dugway Proving Ground in western Utah have produced new data on the chronology and human occupation of late Pleistocene and early Holocene lakes, rivers, and wetlands in the Lake Bonneville basin. We have classified paleo-river channels of these ages as "gravel channels" and "sand channels." Gravel channels are straight to curved, digitate, and have abrupt bulbous ends. They are composed of fine gravel and coarse sand, and are topographically inverted (i.e., they stand higher than the surrounding mudflats). Sand channels are younger and sand filled, with well-developed meander-scroll morphology that is truncated by deflated mudflat surfaces. Gravel channels were formed by a river that originated as overflow from the Sevier basin along the Old River Bed during the late regressive phases of Lake Bonneville (after 12,500 and prior to 11,00014 C yr B.P.). Dated samples from sand channels and associated fluvial overbank and wetland deposits range in age from 11,000 to 880014 C yr B.P., and are probably related to continued Sevier-basin overflow and to groundwater discharge. Paleoarchaic foragers occupied numerous sites on gravel-channel landforms and adjacent to sand channels in the extensive early Holocene wetland habitats. Reworking of tools and limited toolstone diversity is consistent with theoretical models suggesting Paleoarchaic foragers in the Old River Bed delta were less mobile than elsewhere in the Great Basin.
ABSTRACT: A regional synthesis of paleoflood chronologies on rivers in Arizona and southern Utah reveals that the largest floods over the last 5000 years cluster into distinct time periods that are related to regional and global climatic fluctuations. The flood chronologies were constructed using fine-grained slackwater deposits that accumulate in protected areas along the margins of bedrock canyons and selectively preserve evidence of the largest events. High-magnitude floods were frequent on rivers throughout the region from 5000 to 360014 C yrs BP (dendrocalibrated age = 3800-2200 BC) and increased again after 2200 BP (400 BC), with particularly prominent peaks in magnitude and frequency around 1100-900 BP (AD 900-1100) and after 500 yrs BP (AD 1400). In contrast, the periods 3600-2200 BP (2200-400 BC) and 800-600 yrs BP (1200-1400 AD) are marked by sharp decreases in the occurrence of large floods on these rivers.
In the modern record, storms that generate large floods ( 10-year) in the region fall into three categories: (1) winter North Pacific frontal storms; (2) late-summer and fall storms that draw in moisture from recurved Pacific tropical cyclones; and (3) summer storms, mainly convective thunderstorms. Winter storms and tropical cyclones are associated with the most severe floods on the rivers in this study, and are the most probable causes of the paleofloods over the last 5000 years. Floods from both winter storms and tropical cyclones occur when deep mid-latitude troughs steer storm systems into the region. Composite anomaly maps of daily 700-mbar heights indicate that these floods are associated with a low-pressure anomaly off the California coast and a high-pressure anomaly over the Aleutians or Gulf of Alaska. A strong connection exists between the negative phase of the Southern Oscillation Index (often associated with El Niño conditions) and the large floods associated with winter storms and tropical cyclones.
The paleoflood records confirm the existence of centennial-scale variations in the conditions conducive to the occurrence of extreme floods and flood-generating storms in this region. The episodes with an increased frequency of high-magnitude floods coincide with periods of cool, wet climate in the western U.S., whereas warm intervals, such as the Medieval Warm Period, are times of dramatic decreases in the number of large floods. A positive relationship between the paleofloods and long-term variations in the frequency of El Niño events is evident over the last 1000 years. This relationship continues over at least the last 3000 years with warm coastal sea-surface temperatures indicative of El Niño-like conditions.
ABSTRACT: The highest shoreline features of paleo-Lake Malheur are undated gravelly barrier beaches south of Harney Lake that lie ca. 3.5 m higher than the hydrographic outlet of Harney Basin at Malheur Gap (1254 m). The earliest Quaternary record for Lake Malheur consists of occurrences of water-deposited tephra dated to ca. 70,000–80,000 yr ago. The next identified lake interval is dated by shells with ages of ca. 32,000 and 29,500 yr B.P. No dates are available for the terminal-Pleistocene Lake Malheur. Lake(s) were present between ca. 9600 and 7400 yr B.P., although periodic low levels or desiccation are suggested by a paleosol dated as ca. 8000 yr B.P. The lake system probably dried further after 7400 yr B.P., although dates are lacking for the period between ca. 7400 and 5000 yr B.P. Dune deposits on the lake floor are ca. 5000 yr old and indicate generally dry conditions. Fluctuating shallow lakes have probably characterized the last 2000 years. A date of 1000 yr B.P. gives a maximum age for beach deposits at 1254 m, near the basin threshold elevation. Thus, the Malheur Lake system may have drained to the Pacific Ocean by way of Malheur Gap during the latest Holocene.
ABSTRACT: A 200 k.y. chronology of river response to climate-related environmental change has been established for northeast Spain using newly developed luminescence dating techniques. This constitutes the best-documented record of late Quaternary river behavior currently available for the North Atlantic region and enables fluvial stratigraphies to be compared with high-resolution ice core and marine oxygen isotope climate series. Pleistocene and Holocene river aggradational episodes coincide with stadial or neoglacial events, while phases of river incision occur during interstadial or interglacial periods. Alluviation and erosion cycles would appear to track variations in sediment supply controlled by vegetation cover and winter storm frequency.
ABSTRACT: Attempts to increase European biodiversity by restoring rivers and floodplains are based on inadequate data on natural systems. This is particularly the case for NW European rivers because all catchments have been impacted by agriculture and river engineering. If river restoration is to have an ecological, as opposed to `cosmetic' design basis then baseline models are required. However, this poses three questions; (a) what is the natural river-floodplain state, (b) how can it be defined and modelled and (c) can this state be recreated today? The first two questions can only be addressed by using palaeohydrological and palaeoecological data. A second and equally vital consideration is the stability/instability of any restored system to change in external forcing factors (e.g. climate) and in this context it may not be realistic to expect baseline models to provide equilibrium solutions but instead to define process-form domains. Over the last two decades evidence has accumulated that the natural state of lowland rivers in much of NW Europe was multi rather than single thread-braided, anastomosing or anabranching. Until recently our knowledge of floodplain palaeoecology was generally derived from pollen diagrams, which have source-area of problems and lack of taxonomic specificity. The precision and breadth of palaeoecological reconstruction (including richness and structure) has been greatly increased by the use of multiple palaeo-indicators including macrofossils, diatoms and beetles. The dynamics of small to medium sized, low-energy, predeforestation floodplains were dominated by disturbance (windthrow, beavers, etc.) and large woody debris. In order to compare the hydrogeomorphological basis of floodplain ecology, both temporally and spatially, a simple index of fluvial complexity is presented. Palaeoecological and geomorphological investigations have the potential to provide in-depth models of the natural range of channel conditions and sensitivity to external change that can be used to provide a scientific basis for floodplain restoration. There is also the possibility that floodplain-channel restoration may be a valuable tool in the mitigation of future geomorphological change forced by climatic instability.
E. C. Carson, J. C. Knox, D. M. Mickelson (2007). Response of bankfull flood magnitudes to Holocene climate change, Uinta Mountains, northeastern Utah. Geological Society of America Bulletin 119 (9): 1066-1078
ABSTRACT: Long-term variations in Holocene flood magnitude were quantified from the bankfull dimensions of abandoned channels preserved on floodplain surfaces in the northern Uinta Mountains of northeastern Utah. Cross-sectional areas of abandoned channels were reconstructed, and relationships derived from the modern gage records were used to estimate bankfull discharges from bankfull cross-section areas. The results indicate systematic (nonrandom) variations of bankfull floods in the northern Uinta Mountains. Large floods, as much as 10%–15% greater than modern, dominated from 8500 to 5000 calendar yr B.P., and again from 2800 to 1000 cal yr B.P. Small floods, as much as 15%–20% less than modern, characterize the periods from 5000 to 2800 cal yr B.P., and from 1000 cal yr B.P. to near present.
The middle and late Holocene record of bankfull flood magnitude compares well with independent evidence for climatic variation in the area. The early Holocene record indicates that larger than modern bankfull floods coincide with warmer than modern mean annual temperature. We hypothesize that an increased range of magnitude for seasonal solar radiation during the early Holocene favored the accumulation and rapid melting of deep snowpacks in the high Uinta Mountains, thus producing large floods despite warmer mean annual temperatures. The episode of smaller than modern bank-full floods between 5000 and 2800 cal yr B.P. coincides with records of increased forest fire frequency in the northern Uintas. Larger than modern floods from 2800 to 1000 cal yr B.P. coincide with a local decrease in forest fire frequency and evidence for minor local glacial readvances. The decrease in flood magnitudes following 1000 cal yr B.P. corresponds to numerous local and regional records of warming during the Medieval Climatic Anomaly.
Graf, J.B., Webb, R. H., Hereford, R. (1991). Relation of sediment load and flood-plain formation to climatic variability, Paria River drainage basin, Utah and Arizona. Geological Society of America Bulletin 103 (11): 1405-1415
ABSTRACT: Suspended-sediment load, flow volume, and flood characteristics of the Paria River were analyzed to determine their relation to climate and flood-plain alluviation between 1923 and 1986. Flood-plain alluviation began about 1940 at a time of decreasing magnitude and frequency of floods in winter, summer, and fall. No floods with stages high enough to inundate the flood plain have occurred since 1980, and thus no flood-plain alluviation has occurred since then. The decrease in magnitude and frequency of floods appears to have resulted from a decrease in frequency of large storms, particularly dissipating tropical cyclones, and not from a decrease in annual or seasonal precipitation. Suspended-sediment load is highest in summer and fall, whereas flow volume is highest in winter. Fall shows the greatest interannual variability in suspended-sediment load, flow volume, and flood size because climatic conditions are most variable in fall. The relation between sediment load and discharge apparently did not change within the period of sediment sampling (1949-1976), even though the channel elevation and width changed significantly. Annual suspended-sediment loads estimated for periods before and after 1949-1976 show that decrease in suspended-sediment load caused by floodplain alluviation in the Paria River and other tributaries could have been a significant part of the decrease of suspended-sediment load in the Colorado River in the early 1940s.
ABSTRACT: Many river systems in western North America retain a fluvial strath-terrace record of discontinuous downcutting into bedrock through the Quaternary. Their importance lies in their use to interpret climatic events in the headwaters and to determine long-term incision rates. Terrace formation has been ascribed to changes in sediment supply and/or water discharge produced by late Quaternary climatic fluctuations. We use a one-dimensional channel- evolution model to explore whether temporal variations in sediment and water discharge can generate terrace sequences. The model includes sediment transport, vertical bedrock erosion limited by alluvial cover, and lateral valley-wall erosion. We set limits on our modeling by using data collected from the terraced Wind River basin. Two types of experiments were performed: constant- period sinusoidal input histories and variable-period inputs scaled by the marined18 O record. Our simulations indicate that strath-terrace formation requires input variability that produces a changing ratio of vertical to lateral erosion rates. Straths are cut when the channel floor is protected from erosion by sediment and are abandoned—and terraces formed—when incision can resume following sediment-cover thinning. High sediment supply promotes wide valley floors that are abandoned as sediment supply decreases. In contrast, wide valleys are promoted by low effective water discharge and are abandoned as discharge increases. Widening of the valley floors that become terraces occurs over many thousands of years. The transition from valley widening to downcutting and terrace creation occurs in response to subtle input changes affecting local divergence of sediment-transport capacity. Formation of terraces lags by several thousand years the input changes that cause their formation.
Our results suggest that use of terrace ages to set limits on the timing of a specific event must be done with the knowledge that the system can take thousands of years to respond to a perturbation. The incision rate calculated in the field from the lowest terrace in these systems will likely be higher than the rate calculated by using older terraces, because the most recent fluvial response in the field is commonly downcutting associated with declining sediment input since the Last Glacial Maximum. This apparent increase in incision rates is observed in many river systems and should not necessarily be interpreted as a response to an increase in rock-uplift rate.
Abstract not available
ABSTRACT: Storm-driven episodes of gully erosion and landsliding produce large influxes of sediment to stream channels that have both immediate, often detrimental, impacts on aquatic communities and long-term consequences that are essential in the creation and maintenance of certain channel and riparian landforms. Together, these effects form an important component of river ecosystems. In this paper, we describe issues involved in characterizing and predicting the frequency, magnitude, spatial extent, and synchrony of these sediment influxes. The processes that drive sediment fluxes exhibit spatial and temporal variability over a large range of scales. Disregard of this variability can have unanticipated consequences for efforts to quantify process rates, as we illustrate using landslide densities observed for a storm event in western Oregon. Multiple factors interact to create the temporal and spatial patterns of erosional and mass-wasting events that affect stream channels. Fires, in particular, enhance susceptibility to erosional and mass-wasting processes, and thus affect the timing and magnitude of sediment-mobilizing events. We use examples from west-central Idaho to show how fires, storms, and topography interact to create spatially distinct patches of intense erosional activity. We require quantitative descriptions of these controlling factors to make quantitative predictions of how differences or changes in topography, fire regime, and climate will affect the regime of sediment fluxes. The stochastic and heterogeneous nature of these factors leads us to quantify them in probabilistic terms. The effects of future fire and storm sequences are governed in part by the past sequence of events over time frames spanning centuries and spatial extents spanning entire river basins. Empirical characterization of past events poses a considerable challenge, given that our observational record typically spans several decades at most. Numerical models that simulate multiple event sequences provide an alternative means for estimating the influence of antecedent conditions and for quantifying the role of different controlling factors.
ABSTRACT: Starting from traditional ideas on the climatic steering of fluvial system dynamics, it appears that there are different kinds of climatic influences on system dynamics. They vary from direct climatic forcing (like peak precipitation) to indirect (like permafrost) and partial forcing (like vegetation). Vegetation (or its absence), and not directly climate, is considered as the main cause of fluvial incision (or deposition) during temperate (or cold) periods. However, other external factors than climate and non-climatic factors, such as local basin characteristics (like subsoil lithology and relief), express their effects on the fluvial systems by their role in the energy balance of the river catchment. Finally, internal factors in fluvial system evolution (like thresholds and response time) should not be neglected.
ABSTRACT: Channel incision began after 1880 in Douglas Creek, a tributary of the White River in northwestern Colorado. This modern erosion produced a complex series of discontinuous, unpaired terraces below the 1880 valley floor. As many as four of these surfaces were formed below the level of the old valley floor by a process of discontinuous downcutting that apparently was not related to changes of base level, climate, or land use, although the initial incision was probably due to the introduction of large numbers of livestock.
The progress of incision was impeded as large quantities of sediment were flushed from steep tributary valleys into the main channel. Temporary storage and flushing of the sediment by episodic erosion produced a complex post-1900 terrace sequence. This phenomenon may be expected following rejuvenation of areas of high relief and high sediment production, and episodic incision may be a normal part of the erosional evolution of such areas.
ABSTRACT: The floodplain along a 75-km segment of the Brazos River, traversing the Gulf Coastal Plain of Texas, has a complex late Quaternary history. From 18,000 to 8500 yr B.P., the Brazos River was a competent meandering stream that migrated from one side of the floodplain to the other, creating a thick layer of coarse-grained lateral accretion deposits. After 8500 yr B.P., the hydrologic regime of the Brazos River changed. The river became an underfit meandering stream that repeatedly became confined within narrow and unstable meander belts that would occasionally avulse. Avulsion occurred four times; first at 8100 yr B.P., then at 2500 yr B.P., again around 500 yr B.P., and finally around 300 yr B.P. The depositional regime on the floodplain also changed after 8500 yr B.P., with floodplain construction dominated by vertical accretion. Most vertical accretion occurred from 8100 to 4200 yr B.P. and from 2500 to 1250 yr B.P. Two major and three minor periods of soil formation are documented in the floodplain sequence. The two most developed soils formed from 4200 to 2500 yr B.P. and from around 1250 to 500 yr B.P. These changes on the floodplain appear to be the result not of a single factor, but of the complex interplay among changes in climate, sediment yield, and intrinsic floodplain variables over time.
ABSTRACT: Fifteen alluvial sequences in Texas and Oklahoma exhibit the same late Holocene record of channel trenching at 1 ka. The erosion was preceded by slow alluvial sedimentation in most stream valleys, resulting in the formation of a cumulic, organic-rich flood-plain soil previously named the Copan Soil. The soil formed during a period of regionally moister climate, as indicated by pollen spectra, molluscan faunas, vertebrate faunas, sedimentary structures, and high alluvial water tables. At 1 ka, the regional climate changed from moist to dry, coinciding with an episode of channel incision of valley floors throughout the southern Great Plains. Channel trenching occurred simultaneously in both small and large streams in drainage basins of the Arkansas, Red, Trinity, Brazos, and Colorado rivers; the sequences are the first documented example of widespread Holocene incision accompanied by firm evidence for a synchronous change in regional climate.
Macklin, M. G., B. T. Rumsby, T. R. Heap (1992). Flood alluviation and entrenchment: Holocene valley floor development and transformation in British uplands. Geological Society of America Bulletin 104 (6): 631-643
ABSTRACT: The morphology, sedimentary properties, and sequence of recent coarse-grained flood deposits and earlier Holocene alluvial fills were investigated in Thinhope Burn, a small (12-km2 ) catchment in the Northern Pennine uplands, northern England. Twenty-one large flood events are recorded by distinctive cobble-boulder bars, sheets and splays, and boulder berms and lobes. Lichenometric analysis showed that all but one of these floods dated from the mid-eighteenth century. The timing of large floods between 1766 and 1960 corresponds with major hydroclimatic trends evident in northern Britain and northwest Europe over this period. Discharge estimates suggest that flood magnitudes have decreased since the mid-eighteenth century. Channel and flood-plain metamorphosis in late Roman times and in the eighteenth century, following major valley-floor entrenchment (locally as much as 8 m), would appear to have been caused by increased runoff and flood magnitude. This was linked to a shift to a wetter climate with flow augmented by Iron Age and Roman woodland clearance, and drainage of the catchment in more recent times. Results from this study suggest that current models of longer-term Holocene and Pleistocene valley-floor development in the British uplands may need to be re-evaluated.
ABSTRACT: Flood-plain aggradation within the Loup River Basin of central Nebraska was episodic and alternated with incision throughout much of the Holocene. A widespread episode of flood-plain stability, however, occurred about 5700–5100 cal. year BP. The purpose of this paper is to describe the properties of this buried flood-plain at six sites in the basin, to consider why the properties of the buried flood-plain vary from site to site, and to evaluate possible reasons why the Loup River flood-plains stabilized 5500 years ago. Episodic valley-bottom aggradation was common during flood-plain formation at five of the six sites. The radiocarbon ages, particle-size data, and organic-carbon data for the buried flood-plain reveal that valley-bottom aggradation generally slowed between about 5700 and 5100 cal. year BP. Erratic down-profile changes in percentages of sand, clay, and organic matter indicate flood-plain sedimentation and soil formation were often episodic. Sand and clay rarely show a steady fining-upward trend. Organic matter fluctuates with depth; at some sites multiple, incipient A horizons were buried during waning valley-bottom aggradation. At two localities, the buried flood-plain is evident as a clay-rich stratum that must have been deposited in a paleochannel. Flood-plain stabilization between 5700 and 5100 cal. year BP probably occurred in response to the effects of external climate forcing on vegetation and hydrologic changes. Flood-plains of other rivers in the central Great Plains also stabilized at this time, further supporting a climatic explanation for slowing of valley aggradation and formation of a flood-plain at this time. Recognition of buried flood-plains is important to both soil mapping in valleys and to the discovery of cultural resources in valleys.
ABSTRACT: This paper integrates concepts derived from the literature to focus upon interactions between riparian vegetation and river channel dynamics in alpine glacier basins. Discussion of the nature and variability of discharge and sediment regimes of alpine glacier-fed rivers; downstream variations in the physical character of the river channel and corridor; consequent downstream variations in lateral processes; and regional variations in alpine glacier dynamics, lead to the proposal of a conceptual model of proglacial river channel-riparian vegetation interactions under changing climatic conditions.
ABSTRACT: This paper examines the timing, nature and magnitude of river response in upland, piedmont and lowland reaches of the Tyne basin, northern England, to high-frequency (20-30 year) changes in climate and flood regime since 1700 AD. Over this period fluvial activity has been characterized by alternating phases of river-bed incision and stability coinciding with non-random, decadal-scale fluctuations in flood frequency and hydroclimate that appear to be linked to changes in large-scale upper atmospheric circulation patterns. Episodes of widespread channel bed incision (1760-1799, 1875-1894, 1955-1969) result from a higher frequency of large floods (> 20 year return period) and cool, wet climate under meridional circulation regimes. Phases of more moderate floods (5-20 year return period), corresponding to zonal circulation types (1820-1874, 1920-1954), are characterized by enhanced lateral reworking and sediment transfer in upper reaches of the catchment, and channel narrowing and infilling downstream. Rates of fluvial activity are reduced in intermediate periods (1800-1819, 1895-1919) with no dominant circulation regime associated with lower flood frequency and magnitude. The results of this study provide a valuable guide for forecasting probable drainage basin and channel response to future climate change.