Environmental Engineering Reference
In-Depth Information
dry season that occurs in late summer and early fall. Glacial streams and streams
on the east side of the Cascades have a somewhat different pattern. Here, the highest
flows often occur in the spring and extend into the early summer. The lowest flows in
some years occur during cold periods in the winter. In either case, periods of heavy
rainfall or dry weather can create flows that are above or below seasonal averages.
These natural flow variations indirectly affect fish production as a result of changes
in the quantity and quality of instream habitat.
On a shorter time scale, individual storms can rapidly increase river stage in less
than a day. After the storm, the stage declines to a relatively stable level over a longer
period of time, usually days or weeks. In addition to storm events, limited daily stage
changes sometimes occur during sunny weather as a result of snow-melt runoff.
Flows in
regulated rivers
respond to measures taken to improve river channels
so that they can be used more efficiently in the national economic interest. When
properly engineered and constructed, river regulation ensures the creation of favor-
able conditions for navigation and timber rafting, the maintenance of the necessary
water levels at water intake works, the protection of populated areas and agricultural
land from flooding during spring floods and high water, the slow movement of river
sediment, and the smooth flow of water toward the openings of hydraulic engineer-
ing structures such as dams.
BIOLOGICAL IMPACTS OF FLOW FLUCTUATIONS
I
nCreases
In
f
Low
Evidence of biological impacts from rapid flow increases is scarce. Some impacts
associated with rapid flow increases might be more appropriately associated with
high flows. Eggs and alevins (fry) can be killed when gravel scour occurs, and juve-
nile fish may be physically flushed down the river (Rochester et al., 1984). Some
DID YOU KNOW?
River stage
is an important concept when analyzing how much water is moving
in a stream at any given moment.
Stage
is the water level above some arbitrary
point in the river, usually with the zero height being near the river bed, and is
commonly measured in feet. For example, on a normal day when no rain has
fallen for a while, a river might have a stage of 2 feet (baseflow conditions). If a
big storm hits, the river stage could rise to 15 or 20 feet, sometimes very quickly.
This is important because, from past records, we might know that when the
stage hits 21 feet the water will start flowing over its banks and into the base-
ments of houses along the river. How high and how fast a river will rise during
a storm depends on many things. Most important, of course, is how much rain
is falling, but we also have to look at other things, such as the stage of the river
when the storm begins, what the soil is like in the drainage basin where it is
raining (is the soil already saturated with water from a previous storm?), and
how hard and in what parts of the watershed the rain is falling (USGS, 2014b).
