WSP07358

• File Number: 7630.125
• Description: Wild and Scenic - Colorado Wilderness Act - 1991
• State: CO
• Basin: Statewide
• Date: 7/1/1991
• Author: Hill and Platts
• Title: Ecological and Geomorphological Concepts for Instream and Out-of-Channel Flow Requirements - Excerpt From Rivers - Volume 2 Number 3
• Water Supply Pro - Doc Type: Report/Study

OJ2837

TABLE 1
Continued

Instantaneous peak flow
based on 76 years of recorda

63,000

83,500

95,200

117,000

125,000

108,000

Duration of daily mean flow for period of record 1911-1917, 1920-1989:
discharge (ds) equaled or exceeded for indicated percent of time

1%

15%

5%

10%

69,900

29,800

20,700

44,500

70%

95%

80%

90%

4,320

3,410

3,050

3,900

'Weighted skew, 0.500,

ally has not been considered as important
to channel processes as rising limb flows.
Consequently, Rosgen et aL (1986) suggest
decreasing flows by 10% of peak flow each
day, This would allow a regulated stream
to go from peak flow to base flow over a
10-day period. However, natural high flows
seldom operate in this short a lime frame,
Reducing the duration of peak flows may
impair some channel.forming processes
and certain types of vegetation seeding and
growth (Franz and Bazzaz 1977),
In the absence of supporting research,
we recommend that flows be reduced by
no more than 10% of the previous day's
flow, and in most cases a reduction of less
than 10% of the previous day's flow would
be highly preferred. A less than 10% re-

2500

20%

40%

50%

60%

30%

14,500

6,180

4,750

8,IIO

5,330

98%

99.5%

99,9%

99%

2.690

2,330

2,010

2,520

duction would assist in protecting fish from
stranding and provide a longer period of
high flows for vegetation seeding, At high-
er flows, we recognize that the incremental
changes in fl.ow reduction rates are not
greatly different; however, as flows de-
crease there is increasing separation be.
tween drawdown rates. We illustrate these
differences in Figure 2, using three draw-
down rates for the Whitebird site, In prac-
tice, stream flows would not be reduced be-
low some minimum instream flow for fish.
One of the commonly used hydraulic
simulation models for evaluating flood
flows is the HEC-2 model (US. Army Corps
of Engineers 1982), This model utilizes a
step-backwater approach to determine the
velocity and water surface elevation for

Cil 2000
OJ ~
.s VALLEY 0
1500 1....,...:1 RIPARIAN 0
W D
" CHANNEL 0
a: [3
<l: -:. -:'~' FISHERIES 0
I 1000 MEAN 0
(J
en
15
500
0 ",; .......
J F M A M J J A S 0 N D
MONTH

FIGURE 1. Minimum monthly stre;tmflow requirements for fisheriesl ch;tnnel m;tinten;tncel rip;tri;tn
h.ibit;ttl .and nlley m;tintenance in S.ilmon Ri"er;tt Whitebirdl ld~ho.

I~ 204

Rivers' Volume 2, Number 3

July 1991