7378

• UCREFRP Catalog Number: 7378
• Author: Wydoski, R. S.
• Title: Potential Impacts of Alterations in Streamflow and Water Quality on Fish and Macroinvertebrates in the Upper Colorado River Basin.
• USFW Year: 1980.
• USFW - Doc Type: 77-147
• Copyright Material: YES

.l.lU

times as great above the reservoir as below the reservoir. and attri-

buted these differences to daily and seasonal streamflows. He showed

that daily vertical fluctuations of four feet commonly occur tmmediately

below the reservoir and stated that a maximum fluctuation of 1.850 cubic

feet per second may occur in less than one minute. The impact on inver-

tebrates is extremely detrimental especially when over 80 percent of the

120
W
..J
Q.
~
<
(f)
090
a:=
<
o
z
~
,60
(f)
~
(f)
Z
<
C) 30
a:=
o

o
z

stream environment may be dry at low streamflows and severe ice cover

occurs in winter.

Catastrophic drift of chironomid larvae was recorded in the Green

River on 11 June 1965 (Pearson. 1967; figure 4). A sudden increase in

water level of nearly three feet caused an increase in the drift rate of

chironomid larvae. The drift decreased to nearly pre-flood rates after

about 3 hours. however. even though the water level remained high, indi-

cating that the organisms had been dislodged forcibly by the high water

velocity.

These examples demonstrate that changes in reservoir discharges to

a large stream such as the Green River can have a marked effect on bottom

macro invertebrates in the river. The fine review of streamflow patterns

below large dams on stream benthos by Ward (1976) prOVided the following

main points for consideration: (1) each dam must be considered individ-

ually in establishing flow criteria, (2) benthic species composition is

considerably modified by impoundments, (3) the flow regime may enhance

or reduce benthic organisms, and (4) the responses of benthic organisms

to smaller, sublethal effects related to streamflow are presently unknown.

o

0600

---
,
,
, ,....
'" ....
,------

~,
~ ,
,
,
,
WATER ~ "
LEVEL ,

CHIRONOMIDAE
LARVAE

\

1200

1800

2400

HOURS

Figure 4.

Increased Drift Rates of Chironomidae larvae after a Sudden
Increase in Flow, Little Hole, Green River, 11 June 1965

3
....
W
W
~

W
2(f)
<
W
a:=
U
Z
..J
W ~
...
> -
.'
W ..
..J <
a:= c
.
W ...
.... ...
< ..
~ ..
..
..
-
0 ~
,
-
..

Source: W. D, Pearson and D, R, Franklin, "Some Factors Affecting Drift
Rates of Baetis and Simuliidae in a Large River," Ecology vol. 49, no. 1, 1968.