Environmental Engineering Reference
In-Depth Information
A second consideration is: How useful are historical data for predicting existing or future condi-
tions? As discussed previously, rivers and streams are largely inluenced by their watersheds, and
those watersheds are rarely static. For example, as watersheds are developed, the percentage of
impervious areas increases (such as parking lots), so the cumulative impact of all the development
over the watersheds impacts the timing (e.g., time of concentration) and the magnitude of runoff
(see Figure 4.3). Under some conditions, a compelling reason not to use 7Q10 low lows, or peak
lows, based on historical conditions, is that the historical lows are not representative of present
conditions and are not good predictors of future conditions. Also, 7Q10 lows have little meaning in
a highly regulated system, and proposed, planned, and ongoing projects will or can alter the low
regime.
As illustrated in Figure 4.22, depending on the relationship between the groundwater and
surface water levels, the river can be an inluent or “losing” reach, which loses water to the
aquifer, or an efluent or “gaining” reach, which receives discharges from the aquifer (FISRWG
1998). Pumping from aquifers for water supply usually results in a decrease in the groundwater
levels that can change a gaining river to a losing one, impacting riverine low lows. For example,
in the Mississippi Delta, the water level of the Mississippi Alluvia Aquifer, the primary source
of water for agricultural use in the delta, has decreased precipitously in some areas since the
1950s, with a large zone of depression in groundwater levels over the period 1994-2004, as
illustrated in Figure 4.23, and an approximately 27 ft. decline between 1950 and 2008 (Byrd
2011). As a result, signiicant water level declines and the lack of readily available water for
catish production and irrigated agriculture have had a potentially devastating inluence on the
delta's economy (NRCS 1998) and ecology. The reduced river recharge has resulted in declin-
ing base low lows in the Big Sunlower River and other riverine systems (see Figure 4.24) and
has converted some delta streams from perennial to ephemeral or intermittent streams. For
these cases, and for many regulated systems, low characterization estimates based on historical
conditions may not be appropriate. Some of the methods discussed in Section 4.6 can be used
to characterize the degree of hydrologic alternation and environmental impacts. Groundwater,
and surface water-groundwater interactions, also impacts the time and space scales of interest
for water quality management. For example, the extent of the groundwater impact, illustrated in
Figure 4.23, indicates that a basinwide rather than a waterbody-speciic management approach
may be required and that such an approach must consider changes that may occur over decades
rather than months or seasons.
Water table
Water table
(a)
Influent stream reach
(b)
Euent stream reach
FIGURE 4.22 Cross sections of (a) inluent and (b) efluent stream reaches. (From FISRWG, Stream
Corridor Restoration: Principles, Processes, and Practices , Federal Interagency Stream Restoration Working
Group, 1998.)
Search WWH ::




Custom Search