ABSTRACT
During large floods overbank sedimentation is greatest in slack-water areas associated with channel expansions and the mouths of tributaries which are either back-flooded or hydraulically dammed. Slack-water deposits in eastern Washington and central and west Texas provide information on the magnitude, frequency and areal extent of past floods.
The distribution of the slack-water facies of the Pleistocene Lake Missoula Flood deposits throughout the Channeled Scabland of eastern Washington defines the area covered by those floods. The sediments accumulated in basins created by downstream flow constrictions and in back-flooded valleys adjacent to the major channelways. The rhytmically layered slack-water sediments, in many ways analogous to turbidity current deposits, probably resulted from discharge surges during a few great floods.
The lack of soil-profile development on central Texas slack-water deposits is evidence that the accumulation of alluvium at these sites is related to the present hydrologic regimes of the streams. The elevations of the deposits were used to estimate the maximum stage of flooding. The discharge values calculated from the stage estimates approximately agree with the maximum flood of record on these streams.
Tributary canyons of the Pecos and Devils rivers in west Texas are filled with slack-water deposits. The Pecos River deposits are composed of sediment deposited during flooding back up the tributaries from the main stream. In contrast, slack-water deposits in the adjacent Devils River basin are silts deposited during flood surges up the tributaries, interbedded with gravel deposited during floods down the tributary canyons. The more complex hydrologic record in the Devils River slack-water deposits is probably the result of the greater drainage efficiency of its tributary canyons.
The paleohydrologic record derived from studies of slack-water alluvial stratigraphy is useful in assessing the rates of geomorphic change in river226 channels that result from infrequent processes that cannot be measured directly.
