Illinois Natural History Survey - University of Illinois

Effects of Sedimentation on Stream Communities

Excessive input of fine sediment (sand, silt, clay) generally is considered to be the most prevalent form of pollution currently affecting streams and rivers in the United States. Because of the erosive force of flowing water, the presence of fine sediment in streams is an entirely natural phenomenon. In fact, a dynamic balance in streams normally exists between the particle size and amount of sediment transported by a stream, and the discharge and slope of the stream. A variety of human activities can lead to abnormally high rates of sediment input, upsetting this balance and resulting in increased concentrations of sediment in the water column (i.e., increased turbidity) and increased deposition of sediment on the stream bottom. Both of these factors can have serious adverse effects on the biota and ecology of streams.

The major anthropogenic sources of sediment to streams are agriculture (especially row-crop cultivation in floodplains and livestock grazing in riparian zones), forestry (with logging roads contributing far more sediment than other practices, including clear-cutting), mining, and urban development. Of these, agriculture is by far the most significant source of anthro-pogenically derived sediment. It has been estimated that agriculture contributes about 50% of all sediment pollution in the United States.

Several factors make it difficult to study the effects of increased sedimentation on stream communities in natural settings. First, inputs of sediment to streams are often diffuse rather than localized. This makes identification of potentially impacted and relatively unimpacted reaches of stream problematic. In addition, inputs of sediment often may be associated with other sources of pollution (e.g., plant growth nutrients, such as nitrogen and phosphorus, and pesticides). This is especially likely to be true in watersheds where agriculture and urbanization are dominant forms of land use. In such situations, it may be difficult to separate effects of sedimentation from other factors.

Pristine site upstream from impacted areas. Note clear deeper water and abundant woody debris.

Survey scientists recently had the opportunity to conduct research in a setting where such potentially confounding factors should have been relatively unimportant. Numerous streams flow through a U.S. Army installation, Fort Bragg, located in the sandhills region of North Carolina near Fayetteville. In contrast to most streams where effects of sedimentation had been studied, streams at Fort Bragg are naturally soft-bottomed (i.e., largely sand and silt bottom sediments) and slow-flowing. Because of the noncalcareous, predominantly sandy soils that the streams drain, streams in this region have low concentration of dissolved salts, low pH, and are relatively unproductive. Despite their low productivity, these streams support very diverse fish and invertebrate faunas.

A variety of land use practices at the installation were thought to affect rates of sediment input to some streams or stream segments on the base. The most significant sources of sediment were expected to be 1) the construction and intensive use of unpaved, sandy roads, especially where such roads intersected streams, and 2) the virtual devegetation of large tracts of land for paratroop drop zones. Both of these sources result in relatively localized inputs of sediment that, because agriculture is not practiced in watersheds on the base, should not be associated with other potential pollutants (e.g., nutrients, pesticides).

Researchers sampled the benthic macroinvertebrate community at 10 small streams that were hypothesized to vary in sedimentation impacts. At four of the streams, study sites were established upstream and downstream of a potential source of sediment (in all cases a major unpaved road). The three dominant habitats available to macroinvertebrates-- soft-bottomed, midchannel areas, stream banks just below the water surface, and woody material (e.g., root snags, submerged logs)--were sampled at all study sites. In addition to these biotic attributes, a variety of physical parameters were measured at each site to characterize conditions in the stream channel and riparian zone.

The four streams with paired study sites upstream and downstream of a potential source of sediment provided strong evidence that excessive sedimentation was occurring in streams on the base. Striking differences in channel morphology and stream bottom characteristics between upstream and downstream sites could only be attributed to excessive sediment inputs. For example, channel slope was 3 to 8-fold greater at the downstream sites. Such increases in slope are easily accounted for by the qualitative balance that is known to occur in stream channels between stream discharge (Q), sediment discharge (Qs - the mass of sediment moving past a point in the stream per unit time), the median sediment particle size (d50), and stream slope (S): Qs d50 ~QS. This relationship indicates that, if all other factors remain constant, an increase in sediment load (Qs) maintained over a relatively long period of time should result in an increase in slope downstream of the source of sediment. If the left-hand side of the equation does not change (i.e., sediment load and particle size are constant), slope can only increase if discharge decreases, and there were no differences in discharge between upstream and downstream reaches. Upstream and downstream sites also differed in several other important characteristics. Upstream sites were deeper, slower flowing, had bottom sediments comprised largely of silt rather than sand, and had considerably more submersed woody material. Upstream sites appeared to be representative of the "pristine" condition to be expected of small streams in the region. Based on these criteria, the other six streams exhibited varying degrees of excessive sedimentation.

McPherson Creek, a highly impacted stream at Fort Bragg, showing large amounts of suspended sandy sediment

filling the shallow stream channel.

Differences in the structure of the macroinvertebrate community among stream sites were consistent with expectations, based on this physical evidence, that rates of sediment input were substantially elevated at some sites but not others. We observed striking differences among study sites in the number of taxa present; the number of mayfly, stonefly, and caddisfly taxa (insect groups that are relatively sensitive to pollution); and total macroinvertebrate abundance. Two general patterns were evident. First, sites upstream of a main road had significantly greater values for these parameters than sites downstream of the road. Second, two streams, McPherson Creek and Rebel Run, had significantly lower parameter values than all other sampling sites. Physical habitat data suggested that these streams experienced extremely high rates of sediment input.

In most cases, midchannel habitats were more severely impacted by sedimentation than were bank and log habitats. For example, midchannel taxa richness was reduced by over twofold in three out of four downstream sites relative to upstream sites, while taxa richness in the bank and log habitats did not differ between those sites. Midchannel taxa richness was significantly lower in the other six streams than in the upstream sites. The main exception to this pattern was in McPherson Creek and Rebel Run, where all habitat types were adversely affected by sedimentation.

Our results have important implications for the use of biotic criteria in stream monitoring programs. Many monitoring programs focus sampling effort on habitats expected to have the greatest diversity of animals. At Fort Bragg, the bank and wood habitats had by far the greatest biodiversity, even in the relatively pristine sites upstream of sediment sources. Because impacts of sedimentation were greatest in the less diverse, midchannel habitat, these effects would have gone undetected by most monitoring programs. Although we found a diverse and abundant fauna on woody habitat in most streams, the total amount of woody habitat available to macroinvertebrates was severely reduced at sites that experienced excessive sediment inputs. This indicates the importance of assessing the availability of suitable habitat in any attempt to quantify the total impact of sedimentation at a site.

Steven L. Kohler and Daniel A. Soluk, Center for Aquatic Ecology

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