HR: 0800h
AN: H51E-0818    [Abstracts]
TI: Persistence and Geomorphology of Clearwater Side Channels in a Braided River: The More Things Change, the More They Stay the Same
AU: * Curran, J H
EM: jcurran@usgs.gov
AF: U.S. Geological Survey Alaska Science Center, 4230 University Drive, Suite 201, Anchorage, AK 99508, United States
AB: Clearwater side channel lifespans and geomorphic changes are being examined to guide research on importance of side channels to salmon habitat quantity and quality at the river-long scale and implications for land management. Dynamic patterns of channel formation in braided rivers can result in rapid abandonment of channels within the braid plain. When these channels fill with water from hyporheic, regional ground water, or upland tributary sources, clearwater side channels result that might create a stable environment for spawning salmon. To determine the annual to decadal persistence of these side channels, clearwater channels in the 120 km-long Matanuska River in southcentral Alaska are being identified from color differences on a 0.3 m pixel 2006 color orthophoto prepared from 1:24,000-scale aerial photography. Channel identification is being calibrated with field observations of selected channels. This inventory of modern clearwater side channels is being compared to historical conditions from a black-and-white orthophoto prepared from 1949 1:40,000-scale aerial photography, and selected aerial photography for the 1960s and 1980s. Where photo quality limits detection of water clarity, side channel presence or absence is noted. Initial results show that while individual side channels may persist for many years, they have rarely persisted more than a few decades. Analysis of selected, wide braid plain areas shows that multiple water sources may exist for supplying abandoned braid plain channels, an indication that some locations may host clearwater side channels regardless of mainstem position. For example, a clearwater side channel that has been historically used as a spawning site is fed by a tributary known locally as Yellow Creek. This channel is presently being captured by the active braids of the Matanuska River. A comparable clearwater side channel has formed from tributary flow on the opposite bank. However, in 1949, this condition was reversed, with Yellow Creek flowing for 2 km within the braid plain, and the mainstem braids flowing along the opposite bank. This study is also examining the importance of periodic resetting, or fundamental disturbance of the sediment, channel form, and aquatic and riparian vegetation, by the mainstem by measuring the physical and hydraulic characteristics of selected channels used for spawning by salmon. Initial measurements of spring- fed channels show coarse gravel substrates, slow velocities in channels less than 0.5 m deep, and limited opportunities for flooding. These conditions indicate that channel form and substrate is likely little modified from the inherited initial condition, although the biologic activity of redd construction can locally modify bedforms and substrate.
DE: 1813 Eco-hydrology
DE: 1820 Floodplain dynamics
DE: 1825 Geomorphology: fluvial (1625)
DE: 1830 Groundwater/surface water interaction
DE: 1856 River channels (0483, 0744)
SC: Hydrology [H]
MN: 2007 Fall Meeting