7197

• UCREFRP Catalog Number: 7197
• Author: O'Brien, J. S.
• Title: 1983 Yampa River Cobble Reach Morphology Investigation Final Report.
• USFW Year: 1984.
• USFW - Doc Type: Fort Collins, Colorado.
• Copyright Material: NO

14

The dynamics of the system involving the replacement of cobbles as
they are plucked away in short sprints is the genesis of a clean
substrate. Cobble motion is initiated on differential basis, the
smaller sizes moving first until at extreme flows, all except the very
largest sizes may move. Historical tracting of a given cobble would
reveal a slow, tedious progression of the cobbles over the bar with the
movement consisting of short hops and jumps. Cobbles entrained in the -
flow on a riffle, however, will probably be carried through the pool to
the next riffle with eventual deposition on the upstream adverse slope.
Pool constrictions experience greater incremental increases in depth and
velocity which will insure cobble motion to the next riffle. When
relatively large cobbles have deposited in a riffle, the propensity for
other particles to join increases.
Maintenance of dynamic equilibrium requires progressive adjustment
of slope and spatial variation. Bars tend to be energy dissipating
structures that promote overall channel stability. Energy expediture
per unit bed area is equalized with mobilization of cobbles and
localized width and depth adjustment. Channels around cobbles bars are
reformed with failing side slopes and changing widths. At peak flows
vertical accretion of the cobble bar is an example of depth adjustment.
Such accretion, forces additional flow to impinge on the banks and
create side channels of high velocity and unstable beds.
Historical Flow and Flood Frequency Analysis
USGS gaging stations are located at Maybell on the Yampa River and
Lilly on the Little Snake, approximately 40 and 15 miles upstream of
Dinosaur National Monument, respectively. No substantial tributary
inflow or stream losses occur in the intervening reach and therefore,
the gaging station flows represent the inflow to the Monument. On the
average, the USGS discharge computed by combining the discharge at the
stations is slightly higher than that discharge measured at Mathers Hole
(see Figure 6). This comparison is only relative, however, being a
function of the flow travel time, the time of day that the discharge
measurements are made and the magnitude of the unsteadiness of the flow.
The average difference of 6.5% (absolute) between the discharges gives
credibility to stage-discharge relationship at Mathers Hole.
The 1983 Yampa River discharge and the mean annual discharge are
shown in Figures 7 and 8. The mean annual discharge was calculated
using the period 1941-83. The period from 1922-83 constitutes the
entire period of record. The first 20 years of this record were wetter
than the remaining 40, and the 1941-83 period is more representative of
the conditions that exist today in the Yampa River (USGS, John Elliott,
personal communication, 2/83). Table I reveals the marked difference
between the periods. The lower discharge period, 1941-1983, has a mean
water yield of 1,483,700 acre-feet, compared to 1,508,400 acre-feet for
the entire period of record.
Figures 9 and 10 are plots of the five and ten year running average
of the annual volumes of the Yampa River at Deerlodge. The climate was
drier than normal from about 1935 through 1965. The 1920's and 1965-
1975 are wet periods which offset the dry period in the 1930's and 40's.