It is important to determine how urban streams can affect larger waterways, like the Chesapeake Bay, where pollution from upstream sources cause problems in downstream water quality and aquatic habitats. Primary pollutants of concern include nutrients (nitrogen and phosphorus) and sediments, especially in an area with mill dams and legacy sediments. These are associated with the changing of stream macroinvertebrate communities from pollution-sensitive species to pollution-tolerant ones. Stream restoration work, such as bank stabilization and the removal of mill dams, is routinely being employed to attempt to correct the environmental impacts of changes in land use and to reestablish pre-development stream conditions. However, little in-depth monitoring data is available to determine any improvements after the installation of restoration practices.
Nine sites on two branches of the Codorus Creek (East and South) have been monitored for stream health through biological and chemical metrics - nutrients, suspended solids (SS), periphyton, and macroinvertebrates. The sites included locations upstream (reference), at (project) and downstream (impaired) of areas where stream restoration currently is in progress. There were no significant differences in the concentrations of nitrogen, phosphorus, and suspended solids among the sites. This may reflect the continued influence of legacy sediments on downstream water quality or the impact of continuing agricultural activity in locations close to the stream. The large variability in nutrient and sediment concentrations at all of the sites, however, makes it difficult to draw firm conclusions regarding long-term trends in the chemical characteristics of the sites associated with restoration.
Periphyton biomass (chlorophyll a) was higher at the restoration and impaired sites than at the reference sites. This may reflect the influence of nutrients introduced via legacy sediments although the influence of upstream nutrients originating from current agricultural activity cannot be discounted.
Macroinvertebrate diversity did not change significantly at the upsteam reference sites over the course of this study. Sites where stream restoration had been completed, however, showed an increase in macroinvertebrate diversity compared to the same sites before restoration. At these sites there was an increase in the proportion of pollution-sensitive (e.g. Ephemeroptera) organisms relative to pollution-tolerant organisms. This indicates a preliminary biotic response consistent with successful stream restoration to pre-impacted conditions.