Stream channelization and impoundment have reduced heterogeneity in channel morphology and flow characteristics. Channelization constrains channel morphology, removes obstacles to flow, and shortens stream length. These modifications eliminate habitats in overflow areas (such as wetlands and side channels) and in logs and other impediments and accelerate drainage in the channeled sections. Impoundments replace a sequence of turbulent sections and pools behind logs and other obstacles (characterized by rocky substrates and high oxygen contents) with deep reservoirs (characterized by silty substrates and stratification of oxygen content and temperature). These changes in stream conditions eliminate habitat for some species (such as species associated with high flow rate and oxygen concentrations) and increase habitat availability for others (such as species associated with lotic condition and low oxygen concentrations).
The linear configuration of stream systems (i.e., the stream continuum concept; Vannote et al. 1980) makes them particularly vulnerable to disturbances that occur upstream. For example, heavy precipitation in the watershed is concentrated in the stream channel, scouring the channel and redistributing materials and organisms downstream. Fire or harvest of riparian vegetation exposes streams or wetlands to increased sunlight, raising temperatures and increasing primary production, altering habitat and resource conditions downstream, often for long time periods (Batzer et al. 2000a, Haggerty et al. 2004). Industrial effluents, runoff of agricultural materials (e.g., fertilizers), or accidental inputs of toxic materials (e.g., pesticides) affect habitat suitability downstream until sufficient dilution has occurred (S. Smith et al. 1983, Southwick et al. 1995). Eutrophication, resulting from addition of limiting nutrients, substantially alters the biological and chemical conditions of aquatic systems.
Lake Balaton (Europe's largest lake) in Hungary has experienced incremental eutrophication since the early 1960s, when lake chemistry was relatively uniform (Somlyody and van Straten 1986). Since that time, phosphorus inputs from agricultural runoff and urban development have increased, starting at the west end where the Zala River enters the lake. The division of Lake Balaton into four relatively distinct basins draining distinct subwatersheds facilitated documentation of the progression of eutrophication from west to east (Somlyody and van Straten 1986). Dévai and Moldovan (1983) and Ponyi et al. (1983) found that the abundance and species composition of chironomid larvae were correlated with this longitudinal gradient in water quality. The original species characterizing oligomesotrophic conditions have been replaced by species characterizing eutrophic conditions in a west-to-east direction. Similarly, sedimentation resulting from erosion of croplands or clearcut forests or from trampling of stream-banks by livestock alters substrate conditions and habitat suitability for organisms downstream.
Pringle (1997) reported that disturbances and anthropogenic modification of downstream areas (e.g., urbanization, channelization, impoundment, etc.) also affect conditions for organisms upstream. Degraded downstream areas may be more vulnerable to establishment of exotic species that are tolerant of stream degradation. These species subsequently invade upstream habitats. Degradation of downstream areas may restrict movement of upstream species within the watershed, thereby isolating headwater populations and limiting gene flow between watersheds. Finally, degradation of downstream zones may prevent movement of anadromous or catadromous species.
Disturbances to adjacent terrestrial ecosystems affect aquatic species. Davies and Nelson (1994) compared aquatic invertebrate responses to forest harvest within 10 m of streams, 10-30 m of streams, 30-50 m of streams, or unharvested in Tasmania. Densities of aquatic invertebrates were measured at a site upstream of the treatment and at a second site immediately downstream from the treatment. Differences in mayfly (Ephemeroptera) and stonefly (Plecoptera) densities between the two sites were significantly, negatively correlated with width of the riparian forest buffer. Overall, mayfly density declined 62% and stonefly density declined 34% at sites with <30 m of buffer, demonstrating the importance of riparian forest buffers to aquatic species.
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