Selected Water Quality Processes in Rivers and Streams - Hydro-Environmental Analysis: Freshwater Environments - page 138

number of impaired stream miles due to sediments decreased. However, this was primarily a result
of differences in the listing methods and does not imply that those waters that were impaired in
2002 were somehow unimpaired by 2004. In the 2012 list, sediments were the fourth largest cause
of waters not meeting water quality standards.
What is somewhat surprising, given the number of river miles identiied as being impaired, is
that among the state agencies there are few numeric criteria for sediments or siltation. Instead, in
most cases, only narrative criteria are available, such as “The suspended sediment load and sus-
pended sediment discharge rate of surface waters shall not be altered in such a manner as to cause
nuisance or adversely affect beneicial uses.” Therefore, most of the listings are based on qualitative
rather than quantitative assessments. What is recognized though is the national scale of the problem,
where excess sediments “smother stream beds, suffocate ish eggs and bottom-dwelling organisms,
and interfere with drinking water treatment and recreational uses” (USEPA 2002). Some of the
other impacts are illustrated in Figure 5.11.
The leading source of impairments as listed in the 2004 305(b) report was agricultural activities,
such as crop production, grazing, and animal feeding operations (USEPA 2004; Figure 5.12). For
example, some grazing practices with concentrated livestock distributions are known to adversely
affect aquatic, riparian, and upland vegetation, resulting in bank destabilization and stream channel
gullying. Diminished upland vegetative cover and soil compaction also result in increases in rill and
sheet erosion.
The loss of soil from agricultural land has long been recognized as a problem. For example, the
cartoons of Jay Norwood (Ding) Darling in the 1930s brought public attention to a number of envi-
ronmental issues, including the erosion of the topsoil from his native Iowa (Figure 5.13). Ding stated
that “the top soil which goes swirling by in our rivers at lood stage may look like mud to you but it
is beefsteak and potatoes, ham and eggs and homemade bread with jam on it.”
Sediments from agricultural runoff increase sedimentation and other related processes in streams,
which, in turn, impact the stability of stream channels. As discussed in Chapter 2, undisturbed natu-
ral channels will reach an equilibrium condition, as described using the channel evolution model of

e effects of siltation in rivers and streams

Sediment
abrades gills

Sediment blocks sunlight
and reduces the growth of
beneficial aquatic grasses.

Sediment suffocates fish
eggs and bottom-dwelling
organisms.

Sediment reduces available
habitat where fish lay eggs and
other aquatic organisms dwell.

FIGURE 5.11 Impacts of siltation, from water quality assessments, part I. (From USEPA, National
water quality inventory 2000 report, Environmental Protection Agency, Ofice of Water, Washington, DC,
2000b.)