The influence of major flooding on river stability/sediment
The assessment of the erosional impacts of land use on sediment yields and stream ecosystems is often complicated by the episodic nature of major flood events. Such events result in generating large sources of sediment to river systems. How well the landscape and streams accommodate natural events is influenced by modifications of surface and internal drainage by vegetative changes and road systems, conversion from woody to grass/forb riparian communities, direct alterations to stream channels such as channelization, abandonment of floodplains, confinement of river systems, and a large list of other land use changes.
The nature and degree of landscape and river impacts often determines the erosional response to infrequent but major flood events. For example, the 1982 Lawn Lake flood in the Central Rocky Mountain Province in Larimer County, Colorado, caused by the breach of a man-made reservoir, resulted in a flood magnitude 30 times that of the 500 year flood on Roaring River. The Roaring River is a steep gradient, entrenched, low width/depth ratio, large cobble/boulder bed channel (A3a+ stream type, Rosgen, 1994) associated with heterogeneous, unconsolidated materials of glacial till. The geomorphic consequence of this flood on Roaring River were catastrophic. Channel changes resulted in a width of 10-16 feet and depth of 1-2 feet was increased to widths of 70-500 feet and depths from 5-50 feet following the flood (Jarrett and Costa, 1986). The geomorphological nature of these A3a+ stream types is associated with infrequent debris torrents/avalanches that respond similarly to major events. The A3a+ stream types are associated with high energy, high sediment supply channel systems which transport enormous sediment loads as evidenced with the Lawn Lake flood. An extensive alluvial fan of 394,600 yds3 was created at the mouth of Roaring River where seven foot diameter boulders were deposited. The flood waters were then routed into the low gradient Fall River valley downstream of the alluvial fan.
The flood was estimated at 12,000 cfs in Fall River, whose bankfull discharge is approximately 380 cfs. This flood resulted in 1000 cfs/sq. mi., unprecedented in historic times (Jarrett and Costa, 1986). The US Geological Survey estimated this flood to be the largest since the end of the last glacial retreat at the start of the holocene period, 10,000 years ago (Jarrett and Costa, 1986). Fall River is a meandering, gravel-bed alluvial channel in a broad valley with a well-developed floodplain. It classifies as a C4 stream type with a sinuosity of 2, slope of 0.005, D50-35mm, a width/depth ratio of 16 and an entrenchment ratio of 24 (Rosgen, 1994). The riparian vegetation of willows (Salix spp.) is very dense throughout several miles of its length.
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Following the Lawn Lake flood, the dimension, pattern and profile of Fall River remained unchanged. The tight meanders of the river remained intact, with accumulations of sand in the bed that took several years to redistribute (Pitlick, 1985). Fall River is located in Rocky Mountain National Park. This alluvial river was a stable, "reference reach" channel with a well-developed floodplain and had not been extensively grazed. Large Elk populations in the park had been utilizing the willows, but had not changed the density and/or composition of the riparian vegetation. As a result, the stream remained intact despite the high sediment supply and rare, extreme magnitude flood. A photograph of Fall River taken in 2001 is shown in Figure 1, and shows this stable alluvial channel reach downstream of the alluvial fan. Many believe that an alluvial channel would not have maintained the same dimension, pattern and profile following such a catastrophic flood.