Least-Disturbed Streams” (LDS)
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9. Least-Disturbed Stream (LDS) Reaches High-quality stream reaches, sometimes called Stream reaches for the LDS analysis are the “reference streams,” are often used to identify stream segments defined by EPA Reach Files aquatic habitats that are representative of the best version 3.0 or “RF3 stream reaches” (Dewald possible stream condition. Reference streams, and Olsen 1994). Using GIS, each stream reach generally identified using biological assem- was joined with information about its position in blages, ideally have little disturbance from the watershed, local environmental character- human influences, and the biota found there istics and landscape information about the demonstrate natural ecological function. A watershed that drains to each stream reach. drawback to this type of analysis is that only streams that have biological samples can be considered for reference streams. In our study, we used landscape-level attributes of watersheds to select streams with the least amount of relative disturbance in the ACC study area (Figure 9-1). This allowed for an all-inclusive assessment of stream quality which was not limited to locations where biological data have been collected. These streams are referred to here as “Least-Disturbed Streams” (LDS). To select LDS reaches systematically across our © PNHP study region, we chose a suite of 10 landscape- level variables that serve as indicators of overall Limestone Run in Fayette Co., PA is an example of a size 2 watershed quality. This method acts as a Least-Disturbed Stream (LDS). surrogate for time-consuming and costly on-the- ground field visits to individual stream locations. Data Used A key benefit of landscape-level analysis, as opposed to field visits, is that every stream reach Catchment land cover variables in the study area receives the same standardized level of information. Different types of land use (agricultural, urban, forested, etc.) within a watershed often influence The variables used in our study were chosen for water quality, habitat quality, channel condition two reasons: 1) the data were available for the and the hydrology of streams. The land cover entire study area and 2) the variables provide metrics used in this analysis were derived from information about the degree of disturbance and the 1992 National Land Cover Dataset (NLCD), the ecological integrity of stream systems. Ten obtained from USGS (http://seamless. usgs.gov). variables were chosen that represent variations in Using totals of catchment land cover type, point and non-point source pollution, hydrologic proportions of land cover categories were regime, stream connectivity and quality of calculated for the catchment of each stream riparian habitat (Table 9-1). reach. The categories were: • Catchment Urbanization – This category of land Table 9-1. Landscape-level variables associated with RF3 cover was categorized as the sum of “Low stream reaches in GIS used to select least-disturbed stream (LDS) reaches. See text for descriptions of data Intensity Residential”, “High Intensity and sources. Residential” and “Commercial/ Industrial/ Transportation” land cover categories. Catchment Urbanization Riparian Urbanization (impervious) (impervious) • Catchment Agriculture – Land cover in this Catchment Agriculture category was divided into two types: Riparian Agriculture (non-row crop) Catchment Agriculture Riparian Forest Cover 1. Non-row crops – NLCD categories (row crop) “pasture/hay”, “small grains”, “fallow” and # Catchment Road Catchment Forest Cover “urban/recreational grasses”. Crossings # Catchment Point Sources # Catchment Dams 9-1 9-2 Figure 9-1. Least-Disturbed Stream (LDS) reaches for the ACC study area.9-2 See table 9-1 for listing of GIS variables used in the analysis. 2. Row crops – From the “row crop” NLCD in the upstream catchment was enumerated for land cover class. Row crops were used as a each stream reach. Point sources were identified separate land cover class because of their as mines, industrial discharges and permitted potential to contribute different kinds and discharges. Any of these point sources may levels of pollutants to stream systems than non-row crop agriculture. contribute potential toxins to the watershed, degrade water quality and alter stream habitats. • Catchment Forest Cover – This land cover type Although toxin type and amount may differ from represents the sum of different forest types in source to source, the number of point sources can NLCD: “deciduous forest”, “evergreen forest” be an indicator of overall watershed health. and “mixed forest”. Datasets used for point source information (see Riparian land cover variables Appendix A for more information): The types of vegetation and land use in riparian • Mines – USBM Mineral Availability System zones can have localized effects on water (http://minerals.er.usgs.gov/ minerals/pubs) quality, habitat condition, channel alteration and hydrologic regime in streams. The proportion of • Industrial point sources: land cover types in a 100-m buffer for each o Superfund/CERCLIS (EPA Comprehensive stream reach was also calculated using the 1992 Environmental Response, Compensation, NLCD layer (http://seamless.usgs.gov). Riparian and Liability Information System, www.epa.gov/superfund) statistics were not used in the determination of o IFD (Industrial Facilities Discharge, size 4 LDS reaches. www.epa.gov/ost/ basins) o TRI (Toxic Release Inventory Facilities, • Riparian Agriculture – Collective proportion of www.epa.gov/enviro/ NLCD agricultural categories - “row crops”, html/tris/tris_overview.html) “pasture/hay”, “small grains”, “fallow and o Permitted discharges - PCS (EPA/OW urban/recreational grasses”. Permit Compliance System, www.epa.gov/owmitnet/ pcsguide.htm) • Riparian Urbanization (Impervious) – Collective proportion of NLCD urbanization/impervious Number of catchment dams surface categories: “low intensity residential”, “high intensity residential” and “commercial/ The presence of dams can alter natural process of industrial/transportation”. lotic systems such as temperature dynamics, • Riparian Forest Cover – Collective proportion of flow regimes and the transport of nutrients and NLCD forest categories: “deciduous forest”, sediments. In-stream habitats are altered and “evergreen forest” and “mixed forest”. connectivity among aquatic habitats is disrupted. The total number of dams in the catchment Number of catchment road-stream crossings catchment areas was counted for each stream reach. Data on dam locations were acquired from Runoff from roads leads to pollution from the National Inventory of Dams petroleum products, metals and other toxins. In (www.epa.gov/OST/ BASINS). small headwater streams, otherwise intact water- sheds may be disturbed by sediment runoff from LDS Calculations roads. Improperly maintained bridges and culverts at road-stream crossings may lead to Stream reaches were separated into four size habitat and channel alteration. classes (based on watershed area; Table 9-2), so that reference criteria could be assigned to each The number of intersections of roads and streams size class independently. in the upstream catchment were summarized for each stream reach. Roads identified in the In order to identify the Least-Disturbed Streams Census 2000 Tiger line files (http://www.census. (LDS) in the study area regardless of physical gov/geo/www/tiger) were used in the analysis. habitat or ecological regions, we applied one set of criteria to all stream reaches in the region. Number of catchment point sources This was done by finding the cut-off values that showed the top 10% least disturbed streams for The number of point-source pollution discharges each of the 10 different metrics individually (for 9-3 each size class). These cut-off values were then calcareous streams out to determine where the applied to all reaches simultaneously and relaxed least-disturbed examples of this unique stream accordingly until 10% (± 0.3%) of stream type exist. reaches were selected (Table 9-3, Figure 9-1). Crystalline Silicic and Crystalline Mafic Geo- Table 9-2. Size class categories used in the ACC project. logy Streams: These two geology types are found Classes were adapted from those used by The Nature in the southeast corner of Pennsylvania (Figure Conservancy for stream conservation work (Anderson and Olivero 2003). 8-1). Crystalline rocks are formed from solution, such as igneous rock. This is in contrast to sedi- Size Class Watershed Size mentary rock like sandstone, which is formed from the layering and compaction of sediments. 0 – 2 mi2 1. Headwater stream 2 Both crystalline rock types contain certain (0 – 5.2 km ) elements that can leave unique signatures in 3 – 10 mi2 stream water; crystalline silicic rock contains 2. Small stream 2 (5.2 – 25.9 km ) high amounts of silica, while crystalline mafic 11 – 100 mi2 rocks can leave traces of calcium, sodium, iron 3. Mid-reach stream (25.9 – 259.0 km2) and magnesium in surface water. Furthermore, Over 100 mi2 these geology types are found in highly 4. Large streams and rivers (>259.0 km2) populated areas of southeastern Pennsylvania; water quality issues associated with urban Specialized LDS Criteria streams (e.g., stormwater runoff and municipal discharges) may compound or mask the effects Because human settlement, land use and of these unique geologies. pollution patterns can follow regional boundaries, some areas of the study region had © PNHP few or no reference streams identified in the first analysis (Figure 9.1). To identify the best remaining