7196

• UCREFRP Catalog Number: 7196
• Author: O'Brien, J. S.
• Title: Hydraulic and Sediment Transport Investigation Yampa River Dinosaur National Monument 1983 Final Report.
• USFW Year: 1984.
• USFW - Doc Type: Fort Collins, Colorado.
• Copyright Material: NO

44
Sediment Deposition in the Canyon Riparian Zone
Twenty-one cross sections were established early in the spring of
1983 and were monitored later in the fall to discern any and all changes
that had occurred as result of general degradation or aggradation. The
results are reported as an average change in the cross section per foot
of active width of the channel during peak flow in Table VIII. Of the
21 cross sections, 13 aggraded and 8 degraded. The net effect on the
canyon was aggradation with an average of 0.32 feet of deposition per
foot of width of channel per cross section. Of the five cross sections
established in pools above riffles or rapids in the steep reach of river
upstream of Big Joe rapid, three cross sections aggraded, two degraded.
Only the cross section at Tepee displayed a surprisingly large net loss
of sand. The total change in the steep reach represented by these five
cross sections is net degradation. In the cobble reach from river mile
16.5 to 20.5, three cross sections aggraded, two were scoured. The
overall effective change on this reach was negligible. From river mile
10 through 16.5 (from Warm Springs Lake to the spawning site),
characterized as a gravel-small cobble reach with a mild slope and a
riffle-pool sequence three cross sections degraded while six aggraded.
The net change in this reach was apparent deposition. In the sand bed
reach of Warm Srings Lake one cross section aggraded substantially while
the other degraded by an equivalent amount. The upstream cross section,
which is the upstream extent of the backwater created by Warm Springs
rapid, aggraded. The cross section one mile upstream of the rapid
scoured. The single side canyon cross section at Portal Canyon was
completely blocked by sand deposition to a height of six feet. A large
portion of this sand had already been removed by a flushing event of the
side canyon by the end of August. Most of sand deposition or scour was
confined to a relatively small portion of the cross section with the
exception of the last two which were predominately sand bed. The areas
of significant deposition occurred where large eddies formed during peak
flow.
The greatest potential for sediment deposition occurs in the lower
one-third of the canyon, specifically in Warm Springs Lake, the pool
above Warm Springs rapid. The cobble substrate reach retained little
sand deposition after the 1983 runoff season, a year of high sediment
discharge. During 1983 enough sand size sediment (59% of total load)
entered the Yampa Canyon to fill a 300 foot channel, one foot deep, for
over fifteen miles. The cross sections in first third of the canyon had
a net loss of sediment, due in large part to scour at the pool above
Teepee Rapid. The middle third of the canyon experienced no significant
change in the existing channel. The lower one third of the Yampa River
displayed a net increase in sand storage. Although each area sensitive
to sand deposition throughout the canyon was not monitored, enough cross
sections were surveyed to discern the general trend of deposition.
The transition between the riparian and upland vegetation zone
was discernable at fourteen of the twenty-one canyon cross sections.
Various riparian species were identified to mark the transition
including, box elders, cottonwoods and squawbush. Juniper, cactus and
grasses in the upland vegetation zone were also utilized in the seeking