Tm. Allred et Jc. Schmidt, Channel narrowing by vertical accretion along the Green River near Green River, Utah, GEOL S AM B, 111(12), 1999, pp. 1757-1772
The Green River is the longest tributary of the Colorado River. Near the to
wn of Green River, Utah, the Green River narrowed in two discrete phases du
ring the twentieth century. The first phase of narrowing decreased average
width by about 5% and occurred between about 1930 and 1940, when the magnit
ude of 2-yr flood, mean annual discharge, and effective discharge decreased
by about 30%, 28%, and 37%, respectively. During this first phase of narro
wing, saltcedar (Tamarisk spp,), an invading non-native tree, began to esta
blish 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, Furt
her narrowing of an additional 14% occurred after 1959, This latest period
of narrowing began following three successive years when the magnitude of f
loods 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 rive
r, and these alluvial deposits are characterized by horizontal layers, whic
h 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 bas
ed on analysis of more than 2600 discharge measurements made by the U.S. Ge
ological Survey, resurvey of an abandoned measurement site, matches of hist
orical ground-level photography, and analysis of historical aerial photogra
phy within a geographic information system. We have developed analytical te
chniques that permit analysis of width data from U.S. Geological Survey dis
charge measurements where gaging cross sections have adjustable beds and ba
nks.
These techniques allow the spatially rich but temporally poor data from aer
ial photographs to be supplemented with gaging station data, which add grea
t detail about the timing and actual processes of channel narrowing that ca
nnot be determined from aerial photographs alone. Such an analytical strate
gy provides a more complete record of historical channel adjustment than ca
n be obtained by other means.