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Upper
St. Mary's River
Baseline Watershed Assessment
Executive Summary
The St. Mary's River Watershed was identified as warranting further investigation during a 1997 US Army Corps of Engineer (Corps) reconnaissance study of the Lower Potomac River. This study found that, over the next 20 years, the St. Mary's River watershed was likely to experience a rapid increase in population and that this population growth would be concentrated in the upper portions of the watershed. The study also found that this growth would likely result in negative impacts to the aquatic resources within the watershed as a whole. Based on this study, the Corps in conjunction with St. Mary's County, proposed a detailed feasibility study to help manage growth and restore degraded resources within the Upper St. Mary's watershed.
This baseline assessment represents one part of this comprehensive study. The Center for Watershed Protection (CWP) has prepared this assessment to detail the current extent of land development, predict the likely extent of future development, and assess current and future watershed quality based on existing studies and the relationships between land use and watershed health. In addition, CWP will be conducting a stream assessment and a stormwater management retrofit survey within six subwatersheds of the Upper St. Mary's River watershed.The 70 square mile St. Mary's River watershed is located entirely in St. Mary's County, Maryland. The watershed lies within the mid-Atlantic Coastal Plain geologic province. The landscape within the St. Mary's River watershed varies from nearly flat in tidewater areas to gently rolling terrain characterized by low scarps and rises, with deeply dissected stream valleys draining upland areas.
St. Mary's County occupies a peninsula of land in southern Maryland along the western shore of the Chesapeake Bay between the Potomac and Patuxent Rivers. Leonardtown (pop. 1,600), the County Seat and the only incorporated municipality in St. Mary's County, is located outside of the St. Mary's River watershed. Lexington Park (pop. 26,172), although not an incorporated municipality, is the largest community in the county and lies within the St. Mary's River watershed. Lexington Park is centered at the intersection of Three Notch Road (MD 235) and Great Mills Road (MD 246) directly across from the Patuxent Naval Air Station. Smaller crossroad communities are scattered throughout the watershed.
St. Mary's County has adopted a growth management plan (St. Mary's County, 2001) to protect the quality of life and natural resources of St. Mary's County. The plan calls for new growth to be concentrated in areas of existing development around Lexington Park and Leonardtown (i.e., Development Districts), with the majority of the remaining areas included in low density Rural Preservation Districts. The subwatersheds assessed in this baseline report are located within and adjacent to the Lexington Park development district. These subwatersheds include Upper St. Mary's River, Jarboesville Run, Hilton Run, Pembrook Run, unnamed east, and John's Creek.
Current impervious cover for each watershed was estimated from the St Mary's County geographic information system (GIS) data. This data was derived from 1995 digital aerial photography. As St. Mary's County is undergoing development at a rapid rate, development undertaken after 1995 would not be accounted for in these impervious cover estimates.
Year 2000 aerial photographs were obtained from the Farm Services Administration of the U.S. Department of Agriculture to identify post-1995 areas of land development. These areas of land development were identified and their extent measured. Impervious cover coefficients were then used to calculate impervious cover for these areas. This information was combined with the 1995 GIS derived data to arrive at impervious cover percentages that reflect current conditions. A summary of these estimates is presented in Table E-1.
Future impervious cover estimates were projected based on proposed zoning information prepared by St. Mary's County. Impervious cover estimates were developed for each major zoning category contained in the comprehensive plan. These estimates were based on impervious cover coefficients derived in a study of Chesapeake Bay communities (Cappiella and Brown, 2000). The predicted future impervious cover for each subwatershed is provided in Table E-2.
Water quality and biological data have been collected within the St. Mary's River watershed by the Maryland Biological Stream Survey (MBSS), the Department of Natural Resources (DNR) Stream Waders volunteer program, and St. Mary's College as part of the overall Corps study. MBSS sampled benthic macroinvertebrates and fish at a total of nine locations in the St. Mary's River watershed. The headwater streams sampled are currently, or are likely to be, affected by land development activities. In addition to these nine biological and habitat stations, twenty benthic macroinvertebrate and stream habitat stations were sampled in 2000 by the DNR's Stream Waders volunteer monitoring program and analyzed by the DNR. These twenty stations were located throughout the watershed.
The MBSS produced a report entitled Final Data Report: Habitat Quality and Biological Integrity Assessment of Freshwater Streams in the St. Mary's River Watershed (Stranko and Rodney, 2001) that details the findings of this study. This report provides the majority of the biological and habitat information referred to in this baseline report and is included as Appendix D.
The St. Mary's College - St. Mary's River Project sampled water quality, biology (fish and benthic macroinvertebrates), and habitat at 14 stations throughout the watershed in 1999 and 2000. The results obtained were used to augment the information provided in the MBSS report cited above. Overall, St. Mary's college conducted monitoring at 14 locations, eight of which are within the six baseline subwatersheds. The College found an absence of sensitive macroinvertebrate taxa along one reach in the Upper St. Mary's subwatershed, with somewhat low abundance of these taxa along one reach in the Jarboesville Run subwatershed and one reach in the Hilton Run subwatershed. The highest sensitive taxa abundance was found along two reaches in the Upper St. Mary's River watershed.
Fish sampling by the College categorized the samples as Fair to Good. The fish sampling stations located in Pembrook and Hilton Run subwatersheds received a rating of Good. Jarboesville run, near it's confluence with the mainstem of the St. Mary's River and was rated as Good. Two stations on in the Upper St. Mary's River subwatershed received ratings of Fair.
In June 2001, the Center in conjunction with the Corps conducted an intensive survey of the headwater streams draining to the St. Mary's River. Overall this study found the impacts to be most severe in headwater reaches draining older developed areas. The complete results will be reported later this year in a separate stream condition report. This study also involves a detailed field and office assessment of existing and potential future stormwater treatment facilities within the subwatersheds. The purpose is to identify facilities in need of improvement and identify locations for stormwater facilities in developed areas where none currently exist.
Section 6.0 of this report provides a detailed description of the characteristics and conditions within each of the six subwatersheds included in the baseline assessment. The stream condition assessment report will detail the findings of the stream assessment study and build upon the information gathered in preparing this baseline report. A summary of subwatershed characteristics is provided in Table E-3.
Sensitive Streams: These streams typically have a watershed impervious cover of 0 to 10%. Consequently, sensitive streams are generally of higher quality, and are typified by stable channels, good to excellent habitat structure, good to excellent water quality, and diverse communities of both fish and aquatic insects.
Impacted Streams: Streams in this category possess a watershed impervious cover ranging from 11 to 25%, and show clear signs of degradation due to watershed urbanization. Greater storm flows begin to alter the stream geometry. Both erosion and channel widening are clearly evident.
Non-Supporting Streams: Once watershed impervious cover exceeds 25%, stream quality crosses a second threshold. Streams in this category essentially become a conduit for conveying stormwater flows, and can no longer support a diverse stream community. The stream channel becomes highly unstable, and many stream reaches experience severe widening, down-cutting and streambank erosion.