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now extend for a distance of about 20 miles. Between
the levees, the river is narrower and flows deeper and
more rapidly than before, causing more erosion down-
stream. Spawning habitat has been greatly diminished;
and without periodic floods and with less groundwater,
the floodplain flora and fauna have changed. Although
enabling residential and commercial developments on
the floodplain, the levees have reduced the values of
riparian ecosystem services. Attempts have been made
to restore some spawning grounds in tributaries to the
Snake, but at considerable cost.
Like levees, the dikes built across riparian zones for
highways and railroads modify patterns of stream-
flow, thereby altering the riparian ecosystem. Ponding
of water usually occurs upstream, which can be ben-
eficial for waterfowl, but it also elevates the potential
for flooding if culverts and bridges are not well engi-
neered. Downstream from a roadbed, the floodplain
can become stabilized, depending on land use practices
in the area and distance to the next streamflow obstruc-
tion. of course, the most dramatic effects of levees and
dikes occur during periods of high water. notably, dikes
increase the width of riparian zones in some areas, as
does irrigation; levees reduce the width.
crops or retained in the soil. But trade-offs are involved.
First, some of the spring flood water is diverted into
reservoirs, reducing the magnitude of beneficial early
season flows. Second, much of the irrigation water
is transpired or evaporated from croplands, thereby
reducing the amount of water returned to groundwater
or streams. in most western states, evaporation from
reservoirs and cropland (evapotranspiration) account
for more than 90 percent of total water consumption.
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Losses through evapotranspiration constitute a high
proportion of the total irrigation water in drainages
where the amount of water used for irrigation is low,
or where the amount of water required for percolation
back to the stream or groundwater is greater than the
amount of water applied. in other words, if water added
to a meadow or crop is less than the soil profile can
retain against the pull of gravity, then all or most of the
irrigated water will be lost to evapotranspiration and
very little, if any, will flow back to the stream. Many
factors are involved, including climate, the hydraulic
conductivity and water-holding capacity of the soil, and
crop water requirements.
irrigation by flooding has disadvantages as well
as the potential advantage of sustained streamflows
in some watersheds. For example, nutrient leaching
occurs more frequently with flooding, which leads to
fertilizer losses and concomitant nutrient enrichment
(eutrophication) of streams and groundwater. Moreover,
salts that have accumulated in the soil over long
periods are dissolved and transported to the stream
or groundwater, sometimes degrading surface and
groundwater with salinity that is too high for livestock,
cropland, or human uses.
Many farmers now have sprinkler or drip irrigation
systems that are more efficient than traditional
irrigation, because the water is spread more uniformly,
and a smaller portion of the available irrigation water
is transported to a particular field. the same amount
of water can thus be applied to a larger amount of land,
thereby reducing costs. there also is less potential for
the loss of fertilizer by leaching, and consequently
stream eutrophication occurs more slowly. increased
crop production and lower fertilizer requirements
help pay for the required equipment and pumping
costs. However, although using irrigation water more
efficiently has considerable appeal, there is a greater
Irrigation and Streamflow
By whatever means, and whether the water comes from
reservoirs, pumps, or streamflow diversions, irrigation
occurs when resources are available to construct a con-
veyance system and the soils and growing season are
adequate for the crops desired. in Wyoming, the area
with potential for irrigated croplands is small, only
about 2-3 percent of the state. However, the effects are
more far-reaching than the amount of land involved.
one effect is that some streams with formerly yearlong
flows are now dry late in the summer because of with-
drawals upstream, creating ephemeral streams that are
much less valuable for fish, wildlife, and livestock. in
1986 the Wyoming state legislature recognized fishing
and recreation as beneficial uses of streamwater.
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Significantly, flood irrigation can lead to more
streamflow lower on the watershed later in the summer.
this happens because of
return flows,
that is, water
applied by flood irrigation that was not used by the
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