Hyporheic Zones and Mountain Streams
Exploring the factors that control stream-groundwater
interactions and create hyporheic zones in mountain
stream networks and quantifying their effects on
stream ecosystem processes.
The hyporheic zone – areas of the streambed and near-stream aquifers through which stream water flows – has been identified as critically important in stream nutrient cycling, in moderating stream temperature regimes, and in creating unique habitats within streams. The overall objective of this research is to increase understanding of hyporheic zone processes in aquatic ecosystems, including (1) the geomorphic factors driving hyporheic exchange flows, (2) the effect of land-use activities on the hyporheic zone, (3) quantifying the amount of hyporheic exchange flow within stream networks, and (4) the role of the hyporheic zone in stream nitrogen cycling.
Channel Morphology and the Hyporheic Zone:
Channel morphology exerts a primary control on shaping the hyporheic zone in mountain stream networks. While pool-step sequences are one of the most widely described features driving hyporheic exchange, many different morphologic features, and the interactions among them create hyporheic flow nets. Wondzell and Gosseff (in review) provide a detailed descriptions of these relationships. For a broad overview also see “Following a River Wherever it Goes.”
|Fig 1. Complex hyporheic flow paths result from interactions among channel morphologic features as shown in these examples: a steep, 2nd-order step-pool channel (A) and a moderate gradient, 5th-order pool-riffle channel (B). Letters indicate morphologic features driving HEF: S – steps; R – riffles; M – meander bends; B – back channels / spring brooks; I – islands; and T – a steep riffle at the mouth of a tributary (see Kasahara and Wondzell (2003) and Wondzell and Gooseff (in review) for more information).||Fig 2. Any process that rearanges channel morphology can have significant impact on hyporheic exchange. In this example, a major flood and debris flow dramatically altered the planform of lower McRae Creek redistributing large wood, eliminating secondary channels, and stripped gravel bars free of riparian forest trees (see Wondzell and Swanson (1999) for more information).||Fig 3. Movement of water through the hyporheic zone is very slow – taking hours to days to move distances of only a few meters. In this example, the effect of log steps on hyporheic exchange is clearly seen – with tracer labeled stream water (yellow) initially downwelling above log steps (7 & 26 hrs) and then spreading more extensively throughout the width of the valley floor (see Wondzell (2006) for more information).|
Effects of land-use activities and large wood:
Because channel morphology exerts a primary control on shaping the hyporheic zone in mountain stream networks, any process that influences channel morphology also has the potential to influence hyporheic exchange flows Wondzell and Gosseff (in review). Many land-use activities influence channel morphology – from building dikes and stream-side roads that simplify planform geometry to altering inputs of both large wood and sediment through logging related changes in erosion and mass wasting (Wondzell et al., 2009).
Fig 4. In mountainous regions of the Pacific Northwest, floods, landslides, and debris flows episodically reshape stream channels as shown in this matched photo pair (Fig 4A taken before the 1996 flood and debris flow; Fig 4B showing the same site in the summer of 1996). A landslide originated on the ridge top, high above the stream entraining sediment from a road prism spanning a major hillslope hollow. The pictured stream reach is ~2 kilometers from the initial landslide. In this stream reach, the channel downcut approximately 1.5 to 2 meters, shifted several meters to the right, and deposited large wood on the stream bank. None of the large logs visible in the upper photo remained in place; all the large wood in the lower photo is newly deposited.
This site had been the focus of much hyporheic research prior to the flood and debris flow (Wondzell and Swanson, 1996a & Wondzell and Swanson 1996b) allowing us to document how this dramatic restructuring of the channel led to substantial decreases in hyporheic exchange in this stream reach (Wondzell and Swanson, 1999).
|Fig 5. A large-wood removal experiment was initiated at Bambi Ck., in south-east Alaska to study the effects of large wood on channel morphology when isolated from other land use activities. We used these data to model the effects of wood removal on hyporheic exchange. Initially, we saw a substantial reduction in exchange flows with the loss of the step-pool morphology and erosion of sediment from the channel. Without large wood in the channel, there was a slow transition to a pool-riffle morphology. Over the long term, the accumulated changes in channel morphology led to better developed point-bars, increased sediment storage, and increased hyporheic exchange (see Wondzell et al., 2009 for more information).||Fig 6. Changes in the spatial patterns of upwelling and downwelling zones accompanied the changes in channel morphology and the amount of hyporheic exchange flow. These include major shifts in dominant flow paths and the size and number of upwelling and downwelling patches on the streambed as shown in the figure (see Wondzell et al., 2009 for more information).|
Stream Temperature and Nitrogen Cycling:
The hyporheic zone – that the part of the stream extending below the streambed – can be an important component of aquatic ecosystems. The hyporheic zone can provide unique habitats for aquatic organisms, and exchange of stream water through the hyporheic zone exposes transported solutes to unique biogeochemical environments with subsequent impacts on whole stream metabolism and nutrient cycling. Water temperatures in the hyporheic zone are also typically buffered and lagged, with respect to diel changes in stream temperature. As a consequence, upwelling environments are of special interest, because upwelling water has the potential to be thermally or chemically distinct from stream water.
The effects of the hyporheic zone on both temperature and nitrogen cycling are a function of the residence time of stream water in the hyporheic zone. The influence of hyporheic processes on stream water temperatures or stream nitrogen loads is a function of amount of stream water cycled through the hyporheic zone, relative to stream discharge.
|Fig 7. Stream water with very short residence times in the hyporheic zone tends to show the stream’s temperature signature. As residence time increases the diel temperature variation is damped and as residence times approach 24 hours, temperatures of the hyporheic water approaches the daily average stream water temperature.||Fig 8. Key processes in the nitrogen cycle are also a function residence time. Stream water entering the hyporheic zone is well oxygenated, supporting mineralization and nitrification as DOC is metabolized. As available oxygen is used up and the rate of DOC metabolism decreases, areas of the hyporheic zone farther from the stream show a net loss of nitrogen through denitrification (see Zarnetske et al., submitted for more information).|