FLOOD05589

• County: Statewide
• Community: Statewide
• Stream Name: All
• Basin: Statewide
• Title: Ephemeral Streams Hydraulic Factors and Their Relation to the Drainage Net
• Date: 1/1/1956
• Prepared For: State of Colorado
• Prepared By: USGS
• Floodplain - Doc Type: Educational/Technical/Reference Information

EPHEMERAl. STREAA-lS

and progressive pickup of sediment off the stream bed
or from lossof.water. The relative amounts of these
two is not known. We presume that the downstream
increase in sediment concentration measured by us
can be attributed primarily to percolation into the
channel bed, but this needs further verification.
Many ephemeral streams in the study area have de-
graded somewhat since 1860, when a combination of
climate factors and overgrazing set off an epicycle of
erosion. Most of the gullying occurred relatively soon
after this erosion epicycle began. The beginning was by
no means simultaneous in all valleys, for some had begun
to erode in 1860 and others as late as 1915 (Leopold,
1921), Many are still actively cutting, at least in the
headwaters. So, there is some reason t.o suppose that.
the channels measured might still be undergoing some
deepening which would imply a progressive pickup of
debris along the length of the cbannel. To the extent
that this is true, it would account for a downstream in-
crease of suspended-load concentration.
It is well known that arroyo flows gradually lose
water by percolation into the stream bed, but the
amounts of such loss have been studied only in a few
instances (Babcock and Cushing, 1941). These investi-
gators found that the average rate of water seepage into
the channel of a dry wash in Arizona during flash floods
was about 1 foot of water depth per day. R. F. Hadley
(personal communication) observed a flow in Twenty-
mile Creek near Lusk, Wyo., which he estimated as 40
cfs in an upper reach but which was entirely absorbed
by the channel in a distance of 6 miles. This channel
loss is particularly important in the ephemeral channels
in semiarid areas where stream beds are often underlain
by considerable thickness of sandy or gravelly alluvium.
Though it is not possible from our data to allocate to
these two processes proportions representing their rela-
tive importance, the fact remains that it is logical that
suspended-sediment concentration should increase down-
stream in these channels, It should be expected, there-
fore, that other channel characteristics would show some
'tendency to adjust to th is condition. The manner of
adjustment should be showll by the average values of
the exponents representing the change of channel fadors
with discharge, and those values will be summarized and
discussed ill the following paragraphs.
Before summarizing the salient features of the hy-
draulic geometry of ephemeral streams described in this
report, it should be stressed again that the median
values of the exponents relating width, depth, velocity,
and load to discharge are derived from a small number
of measurements, These median values should, there-

15

fore, be considered as merely indicative of magnitude.
With. this qnalification, the data presented here show
that the general principles laid out in Professional Paper
252 find verification in western ephemeral streams as
well as in a single eastern stream, Brandywine Creek,
Pa. (Wolman, 1954a). At cross sections in ephemeral
stream channels the relations of width, depth, and
velocity each to discharge are generally similar to those
found ill the perennial rivers previously studied, as can
be seen by the tabulation below. The basic equations
are listed in the left column, and the main body of the
table presents values of exponents in the equations for
ephemeral channels and for the average river.
Average downstream Average at-a-station
relations relations

Ephemeral
channels

Average
river Ephemeral
channels I channl:'ls

Average
river
channels 1

w=aQII____~__
d~CQf________
v~kQm______ __
L~pQ'_______

b~O. 5 b~O. 5
f~ . 3 f~. 4
m= . 2 m=. 1
j=1.3 j~.8
m m
y= . 7 f . 25

1 Leopold and ~faddock (1953). 2 Unadjusted median values.

b=0.26'
f~ .33'
m= .322
j=1.32

b~ 0,26
b~ .40
b= .34
j=1. 5-2. 0

1'0 summarize, at discharge. of constant frequency in
both perennial rivers and ephemeral streams, width in-
creases downstream as the square root of discharge.
Velocity, however, increases more rapidly downstream
in ephemeral streams, and this tendency is accompanied
by a less rapid increase in depth. This more rapid down-
stream increase in velocity can be interpreted as a re-
sponse to the downstream increase in suspended-
sediment concentration. Indeed, the observed rela-
tions are ill agreement with the principle enunciated by
Leopold and Maddock (1953, p. 26) that "the rate of
increase of suspended-sediment load with discharge is
a function of the ratio:

rate of incrense of velocity with discharge
rate of increase of depth with discharge "

Those authors state (p, 26) the generalization in this
\vay: lifoI' any given value of b * * * the value of j in-
creases with an increase in the m to j ratio." The
agreement with this principle can be visualized by com-
paring the downstream values of these exponents in the
ephemeral channels with those in the average river data,
So, it can be seen that the channel factors in the
ephemeral channels appear to differ from those of the
average river in a manner "thich is in aecord \vith t.he
observed change of suspended-load concentration,