Sediments record the mechanisms of transport and deposition, but the details of the precision and accuracy of interpretations of processes from the preserved record remains ambiguous. To explore this concept in a fluvial system, Draut and Rubin analyzed relations between grain-size progression in suspended sediment and flood deposits from controlled, dam-release floods in the Colorado River through Marble-Grand Canyon. The results revealed that for these simple floods, most deposits show inverse grading that reflects coarsening suspended sediment (a result of fine-sediment-supply limitation). But eddy-scale variability creates some profiles with normal grading, a pattern that does not reflect grain-size evolution in the flow as a whole. The results are interpreted to suggest that systemwide grain-size evolution in modern or ancient depositional systems requires sampling enough deposit profiles that the standard error of the mean of grain-size-change measurements becomes small relative to the magnitude of observed changes. Collectively, the results reveal that, with sufficient sampling, fluvial deposits can faithfully reveal paleo-sediment flux and discharge.
Assessing grain-size correspondence between flow anddeposits of controlled floods in the Colorado River, U.S.A. by Amy E. Draut and David M. Rubin