Regulated Rivers - Hydro-Environmental Analysis: Freshwater Environments - page 68

frequency and magnitude of looding in the area. Bennett and Rhoton (2003) discuss the formation
and removal of the plug.
Channelization and channel maintenance largely impact the physical habitat of streams and riv-
ers. For example, channelization tends to reduce the benthic habitat with the consequent impact of
reducing species diversity. Sedimentation, resulting from alterations of rivers and their watersheds,
is also a major cause of the impairment of rivers and streams (not meeting water quality criteria) in
the United States today, as relected by the number of impairments listed by states in their 303(d)
reports (reported by the states to the U.S. EPA under Section 303(d) of the Clean Water Act [CWA]).
In 2009, sediment/siltation was listed by the U.S. EPA in its national summary tables as the cause
of nearly 6500 impairments, the fourth greatest cause of waterbodies not meeting water quality
standards in the United States. Other impacts of channelization include (Schoof 1980):

• Draining of wetlands

• Cutting off oxbows and meanders

• Clearing of loodplain hardwoods

• Lowering of groundwater levels

• Reducing groundwater recharge from the streamlow

• Downstream looding

Channelization not only impacts the physical characteristics of the stream or river, but also its
chemical and biological characteristics.
Channelization has had many positive impacts, and has been and will continue to be a common
engineering practice. However, in the past, the maintenance of aquatic habitats has not been a major
design goal of channelization projects. In many cases today, however, ecological considerations are
a component of many design studies, which is relected by the number of academic and government
institutions that are including ecohydraulics in their programs.

3.4 WATERSHEDS
The characteristics of rivers and streams are integrally linked with their watersheds. Therefore,
changes in the watershed will have a direct impact on the physical, chemical, and biological char-
acteristics of the receiving water.
The impact of watershed changes may be illustrated by considering the impacts of urbanization.
In natural watersheds, the magnitude, duration, and timing of runoff are controlled not only by rain-
fall but also by the iniltration of water into surface soils, the storage of water in the watershed (such
as depression storage), evaporation, and transpiration by plants. Iniltration and storage in forested
areas, for example, can be quite high, thereby reducing runoff and increasing the time between peak
rainfall and peak runoff, the lag time. With urbanization comes an increase in impervious areas,
such as parking areas, sidewalks, roads, and other areas through which the water cannot iniltrate.
Since more water runs off, the peak low of the runoff hydrograph (plot of low versus time) is
greater. Also, since storage is less, the lag time is also less, as illustrated in Figure 3.29. Therefore,
the natural low hydrograph is altered.
Most cities, counties, and state agencies have stormwater regulations or ordinances that prohibit
increases in the magnitude of runoff for some design rainfall event as a result of a construction proj-
ect. For example, Ordinance Number 2006-7, an ordinance establishing stormwater control in the
city of Starkville, Mississippi, states that “No development shall be undertaken that increases the
rate of surface runoff to downstream property owners or drainage systems.” For traditional storm-
water engineering, postconstruction peak runoff is reduced using some type of storage facility, such
as a detention or retention pond. However, it is the combined effects of all urbanization/construction
activities that are relected in the runoff hydrograph from the watershed to the receiving river or
stream, so the cumulative effect of urbanization is an increase in peak lows and a change in the lag