Environmental Engineering Reference
In-Depth Information
FIGURE 4.17
Potholes in an ancient river bed (Red Rock Canyon, Sedona, Arizona).
and steelhead fry. On cobble substrate, fry (especially steelhead fry) were inclined
to maintain a stationary position over the streambed (i.e., rheotaxis); whereas, over
small gravel, fry swam around, often in schools. When the water surface dropped,
fry maintaining their position became trapped in pockets of water between cobbles,
whereas mobile fish were more included to retreat with the water margin. When
beaching became imminent, fry over cobble substrate retreated into inter-gravel
cavities, where they became trapped. The difference in stranding rate was facilitated
by the flow of water along a receding margin of the stream. On cobble substrate, the
water drained into the substrate, whereas on finer substrates a significant portion of
the water flowed off on the surface.
Fry of some species are more vulnerable to stranding than others. In Washington
State, stranding of Chinook and steelhead fry has been frequently observed.
Although pink salmon fry and chum salmon fry occur in the same rivers, they strand
DID YOU KNOW?
We cannot say that because a stream rises (doubles) from a 10-foot stage to
a 20-foot stage that the amount of water flowing also doubles. When a river
flows out of main river banks and spreads out along the flatter landscape, even
a 1-inch rise in stage can mean a huge rise in streamflow. So, the amount of
water flowing in a stream might double when the stage rises from 5 to 10 feet
of stage, but then it might quadruple when it goes from 15 to 20 feet. A graphic
called a
rating curve
is used to illustrate typical river stage-discharge (stream-
flow) relationships.
