D--048966 - CALFED Bay-Delta Program - State of California

901
Hydrology and Water Resources
tial lots can be 20%-30% impervioussurface (California Divi- latticework, and other structures do nottranspire. So, vegsion
of Soil Conservation 1971b). Larger parcels would have etation conversion eliminates the active removal of soil toolsmuch
smaller proportions rendered impermeable by construc- ture and its transfer to the atmosphere. Soil moisture would
tion. Although impervious area is a small fraction of dispersed remain higher in the absence of transpiration if soil moisture
"ranchette" development, the amount and intensity of con- recharge could take place through whatever covers the soil
version of natural vegetation to other uses, such as orchards, instead of vegetation. Where impervious areas are convineyards,
pasture, ostrich ranches, and Christmas tree farms, structed, recharge of shallow and deep ground water is miniwill
determine the hydrologic impacts. The area occupied by mized. If the total area of limited infiltration is a significant
roads is closely associated with the density of structures. In fraction of a catchment, ground-water levels will decline.
addition, the quality of road location and construction will Stream flow during nonstorm periods that was formerly geninfluence
the potential for adverse effects. Poorly designed erated by seepage from ground water will also decline.
roads for subdivisions are a principal source of sediment in Ground-water pumping for domestic and irrigation supply
Nevada County (Gerstung 1970). can exacerbate the problems of restricted recharge. In some
cases, irrigation return flows may augment summer stream
Channelization flow.
Channelization (forcing streams into engineered waterways) Water Pollution
has been practiced in the Sierra Nevada since the first miners’
ditches for water supply were constructed and rivers were The changes in runoff are closely related to declines in water
confined in wooden flumes while their gravels were exca- quality associated with urban development. Enhanced runvated.
During the mining era and subsequent development off washes various contaminants off roofs, streets, parking
of water resources for hydropower, municipal, and agricul- lots, gutters, horse corrals, and golf courses and into streams.
tural uses, streams were put into artificial channels to get the Diminished base flow increases the concentration of residual
water to another place where it was wanted. Around roads pollution entering after the floods. Urban pollutants include
and towns, the usual objective of channelization is to get water soil particles, nutrients, heavy metals, toxic organic chemiaway
from a place where it is not wanted. Creeks of all types cals such as pesticides, oil and grease, fertilizers, oxygenand
sizes have been relocated, smoothed, and straightened demanding materials such as yard waste, and bacteria and
to get water away from roads and homes as quickly as pos- other pathogens (Terrene Institute 1994). The diversity of
sible. These ditches, canals, and storm sewers enhance the sources makes control difficult, but best management pracflood-producing
effects of general land conversion by rout- tices are being developed and applied to control urban runing
the extra runoff away from the town or road much more off. Development of riparian areas limits opportunities for
quickly than under natural conditions. Peak flows are aug- filtering, uptake, and assimilation of contaminants. The commented
downstream, but that is typically beyond the con- bined effects of changes in runoff regime, water quality, and
cern of the local channelization project. Flooding in Roseville channel structure resulting from urbanization have profound
during January 1995 was a classic example of this phenom- effects on aquatic life. Eliminating infiltration on as little as a
enon. Failure of artificial drainageways and streets to perform tenth of the catchment area led to declines in population of
as expected can also cause damage within the community fish and amphibians near Seattle (Booth and Reinelt 1993).
attempting to control the runoff, as occurred in Cameron Park
in 1982 and 1983 (Soil Conservation Service 1985). At higher Accelerated Erosion
elevations, runoff rates from snowmelt may also be accelerated
where infiltration is limited in significant fractions of a Removal of vegetation, grading, and exposure of bare ground
watershed (Buttle and Xu 1988). allows erosion to increase dramatically, especially during construction.
Freshly cleared land for a new subdivision in
Vegetation Removal Plumas County produced enough sediment in a single intense
storm to kill 80% of the aquatic life in Big Grizzly Creek (Cali-
Other hydrologic impacts of conversion to residential and fornia Division of Soil Conservation 1971b). In Nevada
commercial land uses include reduction of interception and County, more than a third of the total length of streams has
transpiration functions of trees and other vegetation via their been damaged by siltation and stream-bank erosion resultremoval.
All plants intercept and store some proportion of ing from subdivision development (Gerstung 1970). Erosion
the precipitation received. Water retained in the canopy even- rates in the Middle Creek watershed near Shasta City intually
evaporates. Continuous vegetation cover can reduce creased more than twentyfold following urban development
the amount of water reaching the ground substantially, de- (Soil Conservation Service 1993). Residential construction
pending on storm amounts and frequency. Removing the veg- around Lake Tahoe has been a major contributing factor in
etation largely eliminates this function. Whatever replaces the accelerating erosion and increasing nutrient inputs to the lake
plants usually has some interception capacity. However, roofs, (Tahoe Regional Planning Agency 1988).
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