upon similar factors. The optimum regional classification to underlie modeling of sediment 
expectations and response to anthropogenic disturbance will be based on work described in box 
la. Using a combination of empirical data from relatively undisturbed watersheds and models 
describing the physics of sediment supply and transport, we will estimate expected levels of 
bedded sediment fines and embeddedness in stream and river reaches of specific size, slope, and 
location. We will then examine the association between watershed/riparian land use and the 
deviation of sediment concentrations from expected values, using survey data and data from 
more detailed watershed studies. This effort should include interaction with NERL and NRMRL 
to link landuse and landscape processes that may be responsible for delivering sediments. We 
will need to take into account the likelihood that, because of natural disturbances (fire, 
landslides, in-channel scouring due to instream hydrologic modifications), a certain portion of 
the stream or river resource may have fine sediments substantially above or below the mean 
expected value for the region. Therefore, the degree of impairment associated with deviations of 
sediment from expected values is likely to be expressed in terms of statistical probabilities. 
Once the degree of sedimentation is estimated for sample sites, we will examine associations 
between biotic assemblages (algae, macroinvertebrates, fish, rooted aquatic plants), and/or key 
aquatic species or guilds and deviations of sediment from expected values. In most cases, our 
data sets will include sites affected by multiple stressors besides sediment that could potentially 
act upon these aquatic biota. In such cases, a regional plot of sediment concentration versus 
some biotic assemblage characteristic (e.g., % EPT [Emphemeroptera, Plecoptera, Trichoptera]), 
will appear as a wedge-shaped pattern of points, where progressively higher fine sediment 
concentrations are sufficient to limit % EPT numbers, but low concentrations do not guarantee 
abundant EPT because of other habitat or chemical limitations (Terrell et al. 1996). These 
patterns are consistent with a hypothesis that sediment is limiting biota. After demonstration of 
a plausible causal mechanism (from detailed experimental studies) and elimination of other 
plausible explanations for these observations, we will use these kinds of associational data in a 
weight-of-evidence approach to support modeling the effects of bedded sediments on aquatic 
biota. 
For suspended sediments in streams and rivers, we will focus initially on chronic levels of 
suspended sediments, rather than those resulting from episodic events such as those 
accompanying storms. Expected natural levels of background suspended solids will be set on the 
basis of data from flowing waters in basins relatively undisturbed by human land uses and (in 
rivers) historic water clarity data to the extent possible. Regional reference areas could serve this 
purpose. Where no relatively undisturbed waters exist, as for large rivers, we will use historic 
data or reconstructions of fish and/or macroinvertebrate assemblage composition to infer (from 
published tolerance information) pre-disturbance suspended sediment characteristics. In an 
approach similar to that for bedded sediments, we will examine associations between biotic 
and/or key aquatic species or guilds and deviations of sediment from expected values in 
appropriate regional settings. As for bedded sediments, we will seek patterns that are consistent 
with biotic limitation by suspended sediment in a weight-of-evidence approach to support 
modeling the effects of bedded sediments on aquatic biota, supporting this information with 
controlled experimentation or literature reference to establish the suspended sediment levels that 
cause substantial impairment of assemblages, sensitive guilds, or key species. 
70 
