656 pages / Livestock grazing and beaver trapping alter streams hydrologically and
geomorphically leading to declines in the quality and extent of stream-riparian
ecosystems. The influence of reductions in grazing pressure and fluctuating levels of
beaver activity (treatments) on channel capacity was studied at I 08 channel cross-sections,
located on eight headwater streams in Montana and Arizona. Cross-sections
were surveyed two or three times over a two-to-five year period to determine annual rates
of change as a function of treatment. Most cross-sections in the cattle and elk exclosures
and grazed areas showed minimal changes in area(< 10 percent). Large decreases in
cross-section area were observed in reaches with intact beaver dams, especially near the
dams. The beaver ponds reduced channel capacity between 50 to I 00% in most reaches,
compared to< 25% in reaches without beaver ponds. The ponds effectively restored the
hydrologic connection between the stream and valley floor in less than one year. Upon dam failure, channel capacity increased within a year by 40 percent or more as the ponds
drained and sediment eroded.
A conceptual model describing geomorphic and hydrologic response of a drainage
basin to the entry of beavers and then their removal or abandonment was developed,
based on a literature review and field data. The model suggests that the simultaneous
existence of discontinuous arroyos and wetlands, observed by Euro-American
expeditions to the Southwest prior to settlement, may in fact reflect landscapes
transforming due to recent beaver trapping rather than a recent climate shift. Beaver-dam
failures would trigger channelization and thus greater flood magnitudes as water was
more rapidly routed from upper to lower watersheds.
The study suggests that Euro-American trapping and grazing, though temporally
and spatially separated, combined with two recent periods of above-average precipitation
to transform drainage networks in the West and increase stream ecosystem sensitivity to
climatic variability. This transformation pre-dates the installation of stream gages and
the data collection that forms the current basis of our understanding hydraulic geometry
and fluvial processes. Consequently, current hydraulic geometry relationships and our
understanding of stream sensitivity to climatic variability reflect highly disturbed
watersheds and ecosystems, not intact systems. / Version has slight corrections to the original by the author, as specified in separately attached file Corrections_to_Fouty_2003_PhD_20181810.pdf
| Identifer | oai:union.ndltd.org:uoregon.edu/oai:scholarsbank.uoregon.edu:1794/23927 |
| Date | January 2018 |
| Creators | Fouty, Suzanne Catherine |
| Publisher | University of Oregon |
| Source Sets | University of Oregon |
| Language | English |
| Detected Language | English |
| Type | Thesis / Dissertation |
| Rights | Creative Commons BY-NC-ND 4.0-US |
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