HR: 16:50h
AN: H44A-03    [Abstracts]
TI: Floodplain lakes and alluviation cycles of the lower Colorado River
AU: * Malmon, D
EM: dmalmon@usgs.gov
AF: US Geological Survey, 345 Middlefield Rd, MS 973, Menlo Park, CA 94025, United States
AU: Felger, T J
EM: tfelger@usgs.gov
AF: US Geological Survey, 2255 North Gemini Drive, Flagstaff, AZ 86001, United States
AU: Howard, K A
EM: khoward@usgs.gov
AF: US Geological Survey, 345 Middlefield Rd, MS 973, Menlo Park, CA 94025, United States
AB: The broad valleys along the lower Colorado River contain numerous bodies of still water that provide critical habitat for bird, fish, and other species. This chain of floodplain lakes is an important part of the Pacific Flyway - the major north-south route of travel for migratory birds in the western Hemisphere - and is also used by many resident bird species. In addition, isolated floodplain lakes may provide the only viable habitat for endangered native fish such as the razorback sucker, vulnerable to predation by introduced species in the main stem of the Colorado River. Floodplain lakes typically occupy former channel courses of the river and formed as a result of river meandering or avulsion. Persistent fluvial sediment deposition (aggradation) creates conditions that favor rapid formation and destruction of floodplain lakes, while long term river downcutting (degradation) inhibits their formation and evolution. New radiocarbon dates from wood recovered from drill cores near Topock, AZ indicate that the river aggraded an average of 3 mm/yr in the middle and late Holocene. Aggradational conditions before Hoover Dam was built were associated with rapid channel shifting and frequent lake formation. Lakes had short life spans due to rapid infilling with fine-grained sediment during turbid floods on the unregulated Colorado River. The building of dams and of armored banks had a major impact on floodplain lakes, not only by drowning large portions of the valley beneath reservoirs, but by preventing new lake formation in some areas and accelerating it in others. GIS analyses of three sets of historical maps show that both the number and total area of isolated (i.e., not linked to the main channel by a surface water connection) lakes in the lower Colorado River valley increased between 1902 and the 1950s, and then decreased though the 1970s. River bed degradation below dams inhibits channel shifting and floodplain lake formation, and the capture of fines behind the dams has prevented sediment infilling of the lakes. Bed lowering below dams and in artificially confined reaches could potentially dewater floodplain lakes, a process occurring at Beal Lake, a natural lake used for native fish restoration in the Havasu National Wildlife Refuge. Sedimentation near the upstream ends of reservoirs has created large areas of still water. One of the largest, Topock Marsh, is connected to the main channel, restricting its usefulness as a native fish nursery; other backwater areas are confined by bars that isolate standing water at tributaries.
DE: 1820 Floodplain dynamics
DE: 1825 Geomorphology: fluvial (1625)
DE: 1890 Wetlands (0497)
SC: Hydrology [H]
MN: 2007 Joint Assembly