Three main types of flooding in the Carson River basin are documented in this web site:
- Main-Stem Carson River flooding
Flooding of the main channel of the Carson River involves flows that overbank and flood adjacent areas known as floodplains. A floodplain is generally described as a strip of relatively smooth land bordering a river that overflows at time of high water. There are specific geomorphic features that define a floodplain (Leopold and others, 1992). The Carson Valley is broad and flat due to the uplift and subsequent erosion of the Sierra Nevada, Carson, and Pine Nut Ranges as well as deposition of sediment carried by the Carson River at flood stage. Rivers are not static—they migrate or change direction over time—resulting in a layering of coarse and fine sediment over the valley floor. To most observers, the Carson River channel looks more like a creek bed, particularly to those from more humid parts of the country; however, one needs to only look to the headwater area, the Sierra Nevada, to know the source of most of the streamflow originates from the mountain snowpack. Given the broad floodplain that is now the Carson Valley, the Carson River has not needed to incise its channel to accommodate snowmelt runoff but simply uses the floodplain as a discharge surface.
Video:
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Aerial view of Carson Valley during the 1997 flood (30 second video clip).
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- Alluvial-Fan Flooding
Alluvial-fan flooding occurs on alluvial-fan surfaces adjacent to the mountain block which are gently sloping, fan-shaped landforms created over time by deposition of eroded sediment, and are common at the base of mountain ranges in arid and semiarid regions such as much of the western United States (National Research Council, 1996). Flooding on alluvial fans is characterized by high-velocity flows, active processes of erosion, sediment transport and deposition, and unpredictable flow paths. This type of flooding is the result of intense cloudbursts from summer convective storms — common in this river basin. In the Carson River Basin, urban development on alluvial fans occurs primarily on the east and west sides of Carson Valley, in Douglas County, the west side of Carson City, and in Lyon County.
Video:
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Description of Alluvial-Fan Flooding
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- Debris Flows
Debris flows have been documented for the Kings Canyon Creek drainage located in the piedmont foothills west of Carson City and on alluvial fans in the Genoa area in Carson Valley. Unlike water flows where suspended sediment is insufficient to affect how water behaves, debris flows are characterized by a mixture of sediment and water where the flow becomes slurry similar to wet concrete. In steep canyons, debris flows can achieve high velocities (the rate of flow), transport large boulders in suspension, and cause catastrophic damage from impact or burial. (Pierson, 2005).
References:
Leopold, L.B., Wolman, M.G., and Miller, J.P., 1992, Fluvial processes in geomorphology: Dover Publications, Inc., New York, p. 317. Table of contents and ordering information available at <http://store.yahoo.com/doverpublications/0486685888.html>
Pierson, T.C., 2005, Distinguishing between debris flows and floods from field evidence in small watersheds: U.S. Geological Survey Fact Sheet 2004-3142, available online at <http://vulcan.wr.usgs.gov/Projects/FS2004-3142/>
National Research Council, 1998, Alluvial Fan Flooding: National Academy Press, 182 p. Available online at <http://fermat.nap.edu/catalog/5364.html>
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