WSP09464

• File Number: 8220.101.10.D
• Description: Glen Canyon Dam/Lake Powell
• State: AZ
• Basin: Colorado Mainstem
• Date: 5/24/2000
• Author: Hazel et al.
• Title: Sand Deposition in the Colorado River Ecosystem from Flooding of the Paria River and the Effects of the Nov 1997 Glen Canyon Dam Test Flow
• Water Supply Pro - Doc Type: Report/Study

,
.,

NA U Sand Bar Studies

Final Report

;1

Transport of Flood-Derived Sand From the 3-km Reach
Less than 20% of the sand delivered to the Colorado River in August-September 1997, remained
in the 3-km reach by early November when the 1997 Test Flow occurred, The amount of flood-
derived sand in the reach just prior to the 1997 Test Flow was estimated by comparing bed topography
in Segments 3 and 4 on September 27 and November 3 (Table I), Segments I and 2 were not
surveyed in November, Assuming that the decrease in storage was as great as in Segments 3 and 4, the
total volume of August-September Paria River-derived sand remaining in the study reach at the onset
of the 1997 Test Flow was 205,000 m3 (0.35 million Mg), Thus, 36 days after the last 1997, Paria
River flood, the estimated volume of sand remaining in the reach was approximately 12 to 18% of the
preceding cumulative sand input (2.41 :t 0.48 million Mg) in August-September. In other words,
nearly half of the volume of sand supplied by the Paria River during August-September 1997 had been
eroded prior to the start of the 1997 Test Flow,
These results show that sand initially deposited immediately downstream from the mouth of the
Paria River is rapidly transported further downstream within weeks to months, The rapid erosion of
accumulated sediment in the 3-km reach in fall 1997, was similar to the response of the bed at selected
cross sections in the reach to a large influx of Paria River sediment in August 1992, when sand
deposited by this tributary flood was mostly eroded within 4 months (see Figs, 12-20 on pp, 29-34 in
Graf et ai" 1995),

~
~~

"

!-;

~
>.

~;
~
,.
>
.,

,.

"

~;

i'

.....

.

~:

Changes in Suspended Sediment Concentrations and Grain Size

Results from the USGS suspended-sediment and bed material sampling program at the lower

Marble Canyon gage from August 28 to September 18, 1997, have important implications for the rates

at which Paria River sediment inputs were being transported downstream, Suspended sand

concentrations increased at the gage after each tributary flood (Fig, 4), The greatest measured increase

was observed following the September 15 flood, which had the longest flood duration (- 9 hours

above 50 m3/s) of the four 1997, Paria River floods (Fig, 2a), The arrival of Pari a-derived streamflow

on September 16 coincides with a total sediment concentration increase of nearly a factor of twenty-

four (the sample was taken about 3 hours after the peak had reached the gage), from an average

concentration of 0.022% by volume on September 15 to a value of 0,506% on September 16, Silt and

clay concentrations increased by a factor of twenty-seven (from 0,019% to 0.491 % by volume) and

sand concentrations increased by a factor of six (from 0,002% to 0,015% by volume). During this and

other events during the sampling period, rapid increases in suspended-sand concentration were

>,
'^
I':

~
r
"
iIJ
..:

:i"
,,,
~,l._
"'

~

~~;
~:~
r;,_
.''i

~.'

18