FLOOD00235

• County: Statewide
• Community: State of Colorado
• Stream Name: All
• Title: Stream, Riparian, and Wetland Ecology - Class material, Volume 1 of 2
• Date: 9/1/1987
• Prepared For: Students
• Prepared By: Professor Windell
• Floodplain - Doc Type: Educational/Technical/Reference Information

49

Velocity leversal.Concept

Although the behavior of natural llllu'villl streams is not completely
understood there is a concensus that with increased discharge, tbe,water
slope over pools increaaea faster ~haD that over adjacent riffl..-(Kel1er
1980). This process has been termed the hypotheaia of -Velocity leversal.'
The hypothesis is based on the fact that pools at low flow are
characterized by deep slow moving water and riffles by faster shallow water
(Figure 17). However, at high flow the situation is reversed and pools
become areas with the highest velocity of \Jater (Kell,er 1971, Andrews
1976). As discharge increases over a pool-riffle seql~ence. the initial
velocity in a pool is less than that of adjacent riffles. Eventually a
threshold is crossed and at that point the velocity iln the pool may exceed
that in the riffle. Therefore, velocity rllversal fun,ctions to scour out
pools and to deposit coarse bed materials ,)n riffles during hilh
channel-forllling flows. Simply, pools scour at high flow, fill at low flow,
where.. riffles fill at high flow and scour at low fl,l)W. The scour and
fill patterns for each stream permit pools and rifflels to persist over a
wide range of flows and over the long-term.

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Y~'oc1ty reversal (thrtstKtld) concept: data for Dry Creek nNr W1ntel"\,
CellforTIla.

::'J2ure 17.

From E.II. Keller. 1980. The flu\lbl system: selected observations.
pp. 39-46. In A. Sands (ed). IUparian Fore~ts 1n C.I1fornh. their
ecology and con.erv.ticn: A aymposluft. Pub. Mo. 4101. A~rlr.ulture
Sciences Publications, U. of Cauternt_, Berk.ley, CA 94720.

Convergence-Di vergenCt! Cri teri~~

At high channel-forming flow, acour in stre8111 cha,nnels is aSllociated
with convergence of stream flow and depositl.on is correlated with
divergence of flow (Leliavsky 1966). This is known as the convergence-
divergence criterion. In general, there tends to be a, convergenCE' of flow
in pools and a divergence of flow at riffles during high flow
channel-fOrming flood events (Figure 18). With little or no sed1Dlent
transport, at low flow, observed convergence and divergence of fle~ II1&Y not
operate in pools and riffles (Keller 1980). However, during high flow when