A recent publication by the Rocky Mountain Research Station, An Assessment Methodology
for Determining Historical Changes in Mountain Streams by Mark Smelser and John Schmidt
(General Technical Report RMRS-GTR-6) describes ways to analyze long-term stream
channel change to better understand fluvial processes in mountainous regions. The
methods rely on U.S. Geological Survey (USGS) stream gaging station records. The
methods are designed to reconstruct stream channel histories that can be compared
to histories of climate change, stream regulation, and land-use. These comparisons
are useful for linking geomorphic adjustments to natural cycles, rare events, and
land-use activities and thereby allow resource managers to better understand the
susceptibility of mountain streams to flow regulation and land use.
Methodology Image
Data from the USGS stream gaging network is used because:
The use of gaging data has unavoidable limitations. Geographical coverage may not
be as widespread as desirable because gaging sites are situated to provide the best
combination of accessibility, measurement accuracy, and long-term channel stability.
Gaging stations are also frequently located along relatively stable reaches. Thus,
when quantifying geomorphic change at a gage site, it is important to recognize that
while these sites may be more stable than other parts of the same stream; they are
not static or immune to change. For evaluating historical geomorphic change the
most desirable gages are long-term stations whose point of measurement has not moved
and whose bed and banks are adjustable.
Performing this analysis requires seven different files from each USGS gaging station.
All except the recorder tapes are used to some degree in the historical analysis.
1. Miscellaneous working files
2. Discharge measurements
3. Level notes
4. Recorder tapes
5. Station analysis reports
6. USGS water-data reports
7. Statistical analyses of discharge data.
The publication describes the most efficient location and manner of obtaining this
data which are widely disbursed throughout the organizational structure of the USGS.
Since data collection at USGS gages began as early as 1895, most of the field notes
have been archived. Smelser and Schmidt provide much useful information on data
retrieval methods and suggestions for efficient tabulation and compilation to facilitate
the analysis.
The first step in recognizing geomorphic change is to analyze the station’s stage-discharge
relation (rating curve). The rating relation depicts the relation between measured
discharge and water surface elevation measured at the stream gage. The premise of
the analysis is that if the gage datum and streambed elevation remain constant over
time, then either the gage datum or the streambed elevation has changed. For the
analysis to be useful, all of the data have to be corrected to a common datum.
Figure 1 illustrates the history of gage datum changes at the Ashley Creek
gaging station between 1917 and 1996. The figure suggests that substantial adjustment
has occurred over the period of record. The adjustment could be due to aggradation,
degradation, or gage datum change. Station surveying notes are the primary mechanism
for bringing all of the data to a common datum. Based on this information and other
analysis, all gage heights are adjusted to the current gage datum. Once the data
have been rectified to a common datum level, comparisons of changes in rating curves,
streambed elevations, hydraulics geometry, and changes in channel width to a common
standard are possible.
History of Gage Datum Changes
Figure 1. The history of gage datum changes at the USGS Ashley
Creek gaging station relative to the contemporary 1996 gage datum.
Figure 2 illustrates the scatter of mean streambed elevation through time prior to
correcting for datum changes. The data show 5 jumps (arrows) in the data distribution
that are indicative of datum changes.
Plot of the Mean Bed Elevation
Figure 2. Plot of the mean bed elevation at the Ashley Creek gage
without rectifying the gage heights. Figure 3. Plot of the mean streambed elevation
of Ashley Creek for the period of record (1917-1996) using only measurements made
within 50 feet of the gage.
Figure 3 shows a plot of streambed elevations at Ashley Creek through time corrected
to a common datum. All of the measurements were made within 50 feet of the gage.
The plot shows that the streambed aggraded during the early 1920s, was stable for
approximately 38 years, and then degraded rapidly during the early 1960s. The smooth
curve through the data delineates the temporal trend and represents the application
of a Stineman smoothing function.
In additional analysis, the authors examine changes to channel geometry (width, depth,
velocity), changes to channel cross-section form, and changes in channel width.
A common objective is to piece together the various lines of evidence to establish
a story of how and why the channel changed.
Once geomorphic change has been documented and the timing of adjustment established,
the geomorphic and hydrologic histories are compared and contrasted by examining
geomorphic change with respect to hydrographs for the period of record. One outcome
of this comparison is to try to determine possible cause and effect hypotheses for
channel change. Figure 4, which compares streambed elevation to annual peak discharge,
illustrates one such analysis. The plot indicates that changes in the streambed
may be related to changes in the hydrologic regime, but peak flood events are not
directly responsible for streambed elevation changes.
Comparision of Minimum Stream Bed Elevations
Figure 4. Comparison of minimum streambed elevations of Ashley Creek for the period of record to annual peak discharges.
Smelser and Schmidt also advocate the use of detailed geomorphic mapping to assist
with data interpretation and provide specific guidance for mapping. The cover of
the publication shows an example of a detailed geomorphic map prepared for the study.
Mark G. Smelser has an M.S. in watershed science from Utah State University. Dr.
John Schmidt is an associate professor in the Dept. of Geography and Earth Resources,
Utah State University, Logan UT. You may order copies of this publication by sending
your mailing information in label form through one of the following media.: Telephone
(970) 498-1719, FAX (970) 498-1660, or E-mail rschneider/rmrs@fs.fed.us
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