General Technical Report
Monitoring Watersheds and Streams1
Robert R. Ziemer2
1An abbreviated version of this paper was presented at the Conference on
Coastal Watersheds: The Caspar Creek Story, May 6, 1998, Ukiah, California.
2 Chief Research Hydrologist, USDA Forest Service, Pacific Southwest
Research Station, 1700 Bayview Drive, Arcata, CA 95521. (firstname.lastname@example.org)
Abstract: Regulations increasingly require monitoring to detect changes caused
by land management activities. Successful monitoring requires that objectives
be clearly stated. Once objectives are clearly identified, it is important to map out
all of the components and links that might affect the issues of concern. For each
issue and each component that affects that issue, there are appropriate spatial
and temporal scales to consider. These scales are not consistent between and
amongst one another. For many issues, unusual events are more important than
average conditions. Any short-term monitoring program has a low probability of
measuring rare events that may occur only once every 25 years or more. Regulations that
are developed from observations of the consequences of small "normal" storms
will likely be inadequate because the collected data will not include the
critical geomorphic events that produce the physical and biological concerns.
egulations increasingly require monitoring to detect changes
caused by land management activities. In its most useful
form, monitoring is the job of determining whether some
important physical, biological, or social threshold related to some issue
of interest has been crossed. Watershed analysis (Regional
Interagency Executive Committee 1995) is becoming a widely used approach
for identifying the important issues. After the important issues
are identified, it is then time to develop some detailed ideas about
how those issues are affected, both temporally and spatially. Finally, it
is time to decide what to measure, when to measure, where
to measure, and how those measurements will be used to address
the identified issue. The task is to select the appropriate
measurements, at the appropriate times, and in the appropriate places to
determine whether there has been an important change to the issue
being addressed. A monitoring program that fails to incorporate
these first steps is destined to fail. This paper discusses three issues
critical to the design of successful monitoring projects: problem
definition, scale considerations, and limitations of studies in small watersheds.
Mapping the Problem
Successful monitoring requires that the issue of concern be clearly
stated. Once the issue is clear, it is important to map the important
components and links that might affect that issue. For example, a generalized
diagram of some possible important interactions between land use and
"increased flood damage" can be constructed
(fig. 1). In the specific case of the North Fork of Caspar Creek, the only land uses were timber harvest and a
small length of ridge-top roads (Preface, fig. 2, these proceedings), and
the generalized diagram can be simplified to show the potential influence
of these activities (Ziemer, fig. 7, these proceedings).
Figure 1 A generalized diagram of some possible important interactions between land use and
"increased flood damage" in the Russian River watershed of northern California. Thicker arrows depict a greater
relative effect of land-disturbing activity (rounded rectangles) on physical land condition (italics). Only one example,
the relative effect of land-disturbing activity on soil compaction, is shown.
In other watersheds, the mix of land-use activities can be
more complex. For example, in the Russian River watershed,
principal land-disturbing activities are urbanization, agriculture,
roads, grazing, and timber harvest (fig. 1). Each of these activities
can affect storm runoff and routing in different ways. On a
relative scale, urbanization can increase runoff much more than
other activities, because paved roads, parking lots, and roofs
prevent infiltration of water into the soil and result in rapid and
direct runoff to the stream. Agriculture can change soil structure,
increase compaction, reduce infiltration rates, and increase surface
runoff and erosion. Conversion from forest to pasture can result
in substantial changes in watershed hydrodynamics,
including increased runoff and erosion (Reid, these proceedings).
Increased human settlement in flood-prone areas can directly increase
flood damage without a change in the amount of area flooded.
Increased erosion can result in deposition of sediment in stream
channels, increasing the elevation of the channel bed that, in turn,
increases the frequency and amount of over-bank flooding. Further,
alteration of stream channels by levee construction, gravel mining, removal
of woody debris, and reduced floodplain storage can result
in increased flooding.
A monitoring program to assess whether and how flooding
has increased in a watershed will fail unless first there is an
adequate understanding of the potential interactions of various
land-use activities and flood damage. Once these interactions
are understood, monitoring becomes much simpler and
shortcuts become possible. For example, at Caspar Creek, changes in
peak streamflow after logging was linked to changes
in evapotranspiration and rainfall interception (Ziemer,
these proceedings). Consequently, peak streamflow changes could
be predicted adequately by simply tracking the proportion of
the vegetation removed from the watershed each year. In
another watershed experiencing different types of land use, this
shortcut measurement may produce erroneous results, because changes
in evapotranspiration and rainfall interception by vegetation may
have little relationship to the different land use, watershed
condition, and flooding.
There are numerous examples in which a simple index
is purported to link land use to the issue of concern
(fig. 2). Unfortunately, the index shortcut is often adopted too quickly
on the basis of findings by others elsewhere and without
adequate consideration of the local conditions. If, as in the Caspar
Creek example, there is good local evidence that the index (e.g.,
proportion of the vegetation removed) is closely related to the target issue
(e.g., changes in peak streamflow after logging), the index approach
will be successful. However, if the index does not link strongly to
an issue of concern, or the index is not sensitive to changing land
use, then the index approach will fail.
Figure 2 Example of a short cut that uses an index (e.g., equivalent clearcut acres [ECA] or total
maximum daily load [TMDL]) to simplify the complex relationship between land use and "Increased Flood Damage."
The example of increased flood damage (fig.
1) is relatively simple. A more complicated issue is that of "disappearing
salmon" (fig. 3). In this case, land-use activities such as agriculture,
logging, grazing, and urbanization potentially affect only part of
the salmon's life cycle (Ziemer and Reid 1997). An index that
moves directly from land use to disappearing salmon
(fig. 4) without considering the influence of ocean conditions, fishing
(sport, commercial, subsistence), predation (marine, fresh
water, terrestrial), migration blockage (dams, road culverts,
channel aggradation), and additional factors, will probably be
inadequate. Consequently, a monitoring program designed to measure values
of an index of, for example, watershed condition, equivalent
clearcut acres (ECA), total maximum daily load (TMDL), or measures of
the stream channel (pools, woody debris, etc.) will likely not
succeed for predicting annual variations in fish populations, because
the index is unrelated to many of the principal factors that may
be causing salmon to disappear. Figure 3 itself is an
abbreviated description of the numerous components that might be
important to the problem of disappearing salmon. A variety of other
influences could be added to the diagram and each box could be expanded
to more completely display multiple interactions. For example,
the "higher peak flows" box (fig.
3) can be expanded to become figure 1.
Figure 3 A generalized diagram of some possible important interactions affecting "Disappearing Salmon."
Figure 4 Example of a short cut that uses an index (e.g., equivalent clearcut acres [ECA] or total
maximum daily load [TMDL]) to simplify the complex relationship between land use and "Disappearing Salmon."
The object of this exercise is not to develop elegant
textbook diagrams that describe everything that is known about peak
flows or salmon. Nor are these diagrams intended to be
universally applicable. The process of taking the issue of concern and
then developing a map that displays how that issue might be affected
by local conditions is more important than the final map itself.
A conscientious effort to understand the issue requires
integrating information from representatives of many disciplines and
interests. Such an exercise is a learning experience for everyone involved,
and new issues will emerge that will require further consideration.
For example, figure 3 does not consider the effect of hatcheries on
fish genetics and disease. The information and understanding
gained will allow design of a monitoring approach that has a
greatly improved chance of measuring the proper components at
the proper location at the proper time.
As important as it is to determine what, when, and where
to measure, it is equally important to determine what
not to measure. In this way, what the monitoring is and is not intended to
determine will be clear. If the level of understanding is adequate, those
issues that are not to be addressed will not turn out to be essential to
the overall success of the monitoring program. For example, early
in the North Fork phase of the Caspar Creek study in
northern California, we decided to measure those attributes of
streamflow and sediment transport that we believed would be critical to
future forest practice regulation (e.g., suspended sediment and
storm flow). At the same time, we did not expect that other factors, such
as summer low flow, would be as important to decisions
regarding forest practice regulation as would the hydrologic response of
the watershed during storms. Further, it was much more expensive
to measure summer low flow accurately at each tributary in
Caspar Creek because of leakage by subsurface flow through the gravel
in the channel bed. Similarly, early in the study, we decided not
to monitor water chemistry for the same reasons: it was expensive
and we expected that changes in water chemistry resulting from
timber harvest probably would not be sufficient to require changes in
forest practice regulations. Subsequently, however, summer low
flow (Keppeler, these proceedings) and water chemistry (Dahlgren,
these proceedings) were studied in Caspar Creek. Results from
those studies supported our initial guess that changes in summer low
flow and chemical export after logging were minor regulatory issues.
The relevant spatial and temporal scale for each analysis
depends on the specific issue being addressed. There is no one scale that
is appropriate for all issues. Further, there is often no one scale that
is appropriate for even a single issue. For example, a scale that
is considered appropriate within a physical or biological
context might not be considered appropriate within a political or
social context. Failure to recognize these differing views can doom
a monitoring program. Historically, many monitoring programs
have been deficient because the spatial scale was too small and
the temporal scale too short.
Political and Social Scales
Time. Corporations and stockholders consider quarterly profits
and losses to be an important measure of corporate health.
Politicians often focus on election cycles of 2, 4, or 6 years as their measure of
a program's success. Corporate managers who ignore the
quarterly balance sheet or politicians who ignore the next election may
find themselves out of a job. Company- and
government-sponsored monitoring programs, therefore, are often expected to
produce interpretable results within months to a few years.
People have short memories. The more recent the event,
the more likely that it will be considered in planning. The longer
the period between events, the less relevant it appears to daily
life. Consequently, long-term monitoring and planning are
often considered to be more a philosophical exercise than one of
practical value. A flood that occurs once every 50 years is not considered
by most people to be an important threat, unless it occurred last
year. Long-term monitoring programs instituted after a rare event
may fall victim to flagging interest as memory of the event
fades. Differences in time-perception create a tension between
those seeking short-term solutions and those seeking to protect
Space. As the size of an area increases, the perceived level of
importance to individuals tends to decrease. The perceived importance
changes from individual to family, community, city, county, state, and nation.
A similar hierarchy exists within and between organizations
and disciplines. It is not unusual to find that issues of concern
and monitoring programs stop at some political, social, organizational,
or disciplinary boundary, even though such boundaries make no
sense within the physical or biological context of the issue.
Physical and Biological Scales
Time. Relevant time scales vary by issue. Many
environmental evaluations and monitoring programs are too brief to
adequately reflect the patterns of response that are important to an issue.
Data from such evaluations are almost always insufficient to
identify even trends of change unless the impact is rapid and of
large magnitude. Even in the case of a large, rapid response,
abbreviated time scales for analysis often make it impossible for the
long-term significance of the impact to be evaluated. In the case of
sediment production and movement, a large infrequent storm may
be required to produce significant erosion. Then, a number of
large storms might be required to move the sediment from its point
of origin to some location downstream. Both the erosion event and
its subsequent routing result in a lag between the land
management activity and its observed effect, particularly in large
watersheds (Swanson and others 1992). As a classic example, Gilbert
(1917) described the routing of sediment produced by placer mining
in California during the 1850's. The fine-grained sediments
were transported downstream within a few decades, but the
coarse-grained sediments are still being routed to the lower
Sacramento River, nearly 150 years after mining ceased.
A migratory species might depend on local habitat only
several weeks out of a year. The appropriate analysis for this species
would focus on whether past, present, and proposed management
actions affect that specific habitat for those periods of occupation
each year. Activities that affect the habitat only when the animal is
absent would not be relevant. Long-lived and nonmigratory species
may require an analysis that evaluates the effects of
management activities over all seasons for several decades, or perhaps centuries.
For many issues, unusual events are more important
than average conditions. For example, the morphology of
mountainous channels and much of their diversity in aquatic habitat are
shaped by infrequent large storms. If a geographically isolated
population of a nonmigratory resident species is removed by an unusual
event, the species may not be able to reoccupy the site, even if prior
and subsequent habitat conditions are perfect. Any
short-term monitoring program has a low probability of measuring rare
events that may occur only once every 25 years or more. Regulations
that are developed from observations of the consequences of
small "normal" storms will likely be inadequate because the
collected data will not include the critical climatic or geomorphic events
that produce the physical and biological concerns.
Space. Relevant spatial scales for analysis and monitoring also
vary by issue. For example, the appropriate area in which to monitor
the quality of a small community's water supply is defined by
the boundary of the watershed supplying that water and the system
by which the water is delivered to the consumer. In contrast, to
evaluate the causes of "disappearing salmon"
(fig. 3) would require considering those factors that influence the salmon's life
cycle, including both freshwater and ocean habitats. The affected
area might encompass several states, include several large river
basins, and extend offshore from Alaska to southern California.
The size of disturbance units often corresponds to the size
of small watersheds. Consequently, at any time it is possible to
find some entire small watersheds that have been completely
disturbed, some small watersheds that have not been disturbed for many
years, and a number of small watersheds in varying stages of
recovery from past disturbances. In contrast, only a small proportion of
a large watershed is likely to have been disturbed at any one
time, whereas the remainder of that watershed either has not
been disturbed or is in various stages of recovery from past
activities. Consequently, attempts to detect the effects of land use by
observing the response of large watersheds have often been
unsuccessful because large watersheds tend to represent a homogenization
of land disturbances, with each large watershed having
relatively similar management histories.
Temporal and spatial variability makes detecting
change difficult. Variability between adjacent watersheds
generally increases with increasing watershed size. For example,
the coefficient of variation (cv; ratio of standard deviation to mean)
of peak flows in the 25-ha tributaries of Caspar Creek was about
half (cv = 0.125) of the coefficient of variation between the roughly
450-ha South Fork and North Fork watersheds
(cv = 0.261). Hirsch and others (1990) reported that annual floods in large watersheds
often have a coefficient of variation of one or more. This means
that, given equal effort, a change is more likely to be detected in
small watersheds than a similar magnitude of change in large
watersheds. One reason for this increased variability is that the larger
the watershed, the more likely that rainfall amounts and intensities
will vary within and among watersheds for any given storm. In
addition, disturbances in large watersheds are more variable because
of multiple types of land use, differing amounts of area disturbed
in any given year, and differing character of the land being disturbed.
For each issue and each component that affects that
issue, there are appropriate spatial and temporal scales to consider.
These scales are not consistent between and amongst one another.
For example, the duration and intensity of rainfall that produces
the largest flood peaks vary with watershed size. The largest peaks
in the 0.15-km2 Caspar KJE tributary result from a
"saturated" watershed that then receives intense rains lasting several
hours, whereas those in the 275-km2 Noyo River require rain storms
lasting several days, and those in the
9,000-km2 Eel River require prolonged rains of a week or longer. Consequently, the largest floods in a
large watershed often do not correspond to the largest floods in
To characterize, for example, stormflow and
sediment discharge, measurements must be made more frequently in a
small watershed, because of short response times, than in a
large watershed. However, for equal precision, measurements must
be obtained at more locations in a large watershed, because of
greater spatial and temporal variability, than in a small watershed.
The Role of Small-Watershed Studies
What They Cannot Do
Observations of the response of small watersheds to changing
land use cannot be accurately extrapolated to predict the response
of large watersheds to the same changes in land use, because
the processes of streamflow generation and routing are not
represented in the same proportions. The hydrologic responses of
small watersheds are governed by hillslope processes that are sensitive
to land use practices. In contrast, the hydrologic responses of
large watersheds are governed primarily by channel form and
network pattern (Robinson and others 1995), which are less likely to
be affected by land use practices outside of the channels. Runoff
and sediment from small tributaries are damped, lagged,
and desynchronized as they move downstream into progressively
larger watersheds (Hewlett 1982).
Small-watershed studies should be considered case studies
in which a few selected land-use practices are applied and are
then "tested" by a discrete, but uncontrollable sequence of storm
events. It is rare to find replication in small-watershed studies. First,
small-watershed studies are expensive and time consuming. This
results in very few watersheds being selected for study. Second, it is
difficult to find several watersheds that have comparable conditions to
allow for replicated treatments. Third, and most importantly, it is
not possible to replicate the size and sequence of storms to which
the watersheds are subjected either before or after disturbance.
Most small-watershed studies have experienced the misfortune of
having no large storms in the before-disturbance, after-disturbance, or
both data sets (Wright 1985). The Caspar Creek study has been
fortunate to experience relatively large (20-year) storms in both the
before-logging and after-logging periods. However, during 36 years
of study, Caspar Creek has still not experienced a truly
large geomorphically significant flood.
The response of small watersheds to one type of land use,
such as logging, cannot be used to predict the response of that
watershed, or a different watershed, to another practice, such as
agriculture, grazing, or urbanization. Each practice affects the components
of watershed response differently. For example, there are
few agricultural areas or practices that would produce runoff
from rainfall that is comparable to that generated from a steeply
sloping logged hillslope. Conversely, there are few forested areas or
forestry practices that would produce runoff from rainfall that is
comparable to that generated from plowed agricultural lands.
What They Can Do
Small experimental watersheds such as those at Caspar Creek,
H.J. Andrews (Oregon), Coweeta (North Carolina), Hubbard
Brook (New Hampshire), and Loquilla (Puerto Rico) permit
detailed studies of physical and biological interactions in a
relatively controlled environment. Experimental disturbances can be
imposed at a temporal and spatial scale that allows the researcher a chance
of correctly identifying cause and effect. Further, although
small-watershed studies are only case studies, they can establish
some sideboards on the more outrageous claims that appear now
and then. It is not unusual to hear claims that "logging will dry up
the streams and springs" or "logging will produce devastating
floods" or "logging does not increase landslides or stream sediment loads."
The Caspar Creek studies have shown that none of these claims
are true for the conditions found at Caspar Creek. By
merging information from similar studies at other locations,
generalizations can be made concerning how small watersheds function
and respond to land management under varying climate, geology,
and vegetation (e.g. Lull and Reinhart 1972, Hewlett 1982, Post
and others in press).
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