Abstract

The Green River is the longest tributary of the Colorado River. Near the town of Green River, Utah, the Green River narrowed in two discrete phases during the twentieth century. The first phase of narrowing decreased average width by about 5% and occurred between about 1930 and 1940, when the magnitude of 2-yr flood, mean annual discharge, and effective discharge decreased by about 30%, 28%, and 37%, respectively. During this first phase of narrowing, saltcedar (Tamarisk spp.), an invading non-native tree, began to establish itself in the study area, but botanists of that time did not describe the tree as abundant. Channel width was stable in the 1940s and 1950s even though saltcedar were becoming already abundant on the river's banks. Further narrowing of an additional 14% occurred after 1959. This latest period of narrowing began following three successive years when the magnitude of floods was less than the present 1.5-yr recurrence flood and when saltcedar were already abundant along the river. The deposits that comprise the banks of the narrowing Green River are composed of the suspended load of the river, and these alluvial deposits are characterized by horizontal layers, which indicate that they formed by vertical accretion. A mechanism is proposed to explain the coarsening-upward sequence of beds found in these vertically accreted deposits.

These changes in the channel of the Green River are based on analysis of more than 2600 discharge measurements made by the U.S. Geological Survey, resurvey of an abandoned measurement site, matches of historical ground-level photography, and analysis of historical aerial photography within a geographic information system. We have developed analytical techniques that permit analysis of width data from U.S. Geological Survey discharge measurements where gaging cross sections have adjustable beds and banks. These techniques allow the spatially rich but temporally poor data from aerial photographs to be supplemented with gaging station data, which add great detail about the timing and actual processes of channel narrowing that cannot be determined from aerial photographs alone. Such an analytical strategy provides a more complete record of historical channel adjustment than can be obtained by other means.

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