8252

• UCREFRP Catalog Number: 8252
• Author: Allred, T. M. and J. C. Schmidt.
• Title: Channel Narrowing of the Green River Near Green River, Utah
• USFW Year: 1999.
• USFW - Doc Type: History, Rates, and Processes of Narrowing.
• Copyright Material: NO

24

However, our data indicate that vertical accretion niay elevate f100dpiain

surfaces until inundation rarely occurs (Fig. 16). Vertical accretion during large

flooding events, can elevate floodplain surfaces considerably, leaving them at

higher elevations than would be predicted using long-term effective discharge

calculations.

In order to gain additional insight into the process of inset floodplain

formation, channel width at a lower range of discharges was plotted over time

.
(Fig. 18). Measurements made at lower discharge are more sensitive to

emplacement of low-elevation deposits (see Fig. 6). It is clear that bank-

attached bar emplacement occurred many times betWeen 1930 and 1948. In

fact, a bar persisted for several years in the early 1930s. But, subsequent

- flooding consistently scoured the bar and returned the channel to the pre-

emplacement condition. Bar emplacement occurred again in the late 1950s;

however, low-magnitude floods in subsequent years, due to both natural

climatic variation and Flaming Gorge Dam operations, did not scour this bar.

These low-magnitude floods apparently allowed vegetation to establish and

develop a sufficiently dense root structure to stabilize the bar and prevent
subsequent scour. Beginning in 1959, the added roughness of saltcedar at the
channel margin probably decreased flow velocity and increased the rate of

vertical accretion atop the newly form~d emergent bar. The floods of the 1980s

further elevated the bar surface to a level that has not been subsequently

overtopped.