Open-File Report 99-219 contents page

TIDAL POTOMAC RIVER AND POTOMAC ESTUARY SEGMENTATION AND COVERAGE BY SUBMERSED AQUATIC VEGETATION

The tidal Potomac River and Estuary extends 183 km from Little Falls near Chain Bridge in Washington, D.C., down to the river's mouth at the Chesapeake Bay. For its studies, the CBP has divided the Potomac River and Estuary into three segments by salinity regimes -- tidal fresh, oligohaline and mesohaline. Before 1997, the CBP defined these segments as TF2, RET2 and LE2, respectively, but they have since been redefined and renamed POTTF, POTOH, and POTMH, respectively. (Table 1 summarizes the various abbreviations used throughout this report.) In this report, data are reported by the three segments TF2, POTOH, and POTMH, as shown in Figure 1. Differences in realignment between the oligohaline (RET2 versus POTOH) and mesohaline (LE2 versus POTMH) segments were significant. Realignment within the freshwater tidal regime (TF2 versus POTTF) consisted primarily of the exclusion of two creek areas from the TF2. This designation of POTTF impeded historical comparisons, so that TF2, rather than POTTF, was used in this study.

For USGS study purposes, the tidal fresh segment, TF2, has been further subdivided into two segments, an upper tidal fresh (UTR) and lower tidal fresh (LTR) segment. (Carter and Rybicki, 1986; Carter and Rybicki in Batiuk and others, 1992.) Studies have indicated that historical patterns of SAV growth in UTR and LTR segments have been different (Landwehr and others, 1997). With respect to the TF2 versus POTTF designation, the POTTF designation would exclude SAV-contributing areas from Piscataway Creek in the UTR and from Mattowoman Creek in the LTR. The areal coverage of SAV in UTR and in LTR sum to that in the TF2; hence, to facilitate time series comparisons, we have chosen to retain the TF2 segment.

The growing season for SAV in the Potomac River extends from April through October. Using aerial photographs and on-site assessments, the Virginia Institute of Marine Science (Orth and others, 1997; and internet site http://www.vims.edu/bio/sav/ ) provides estimates of the annual SAV coverage in hectares for each of these CBP segments. USGS researchers, working with researchers at the Virginia Institute of Marine Science, have further partitioned the TF2 SAV coverage into coverage within the UTR and LTR segments. A summary of SAV coverage by river segment for the period 1983 through 1997 is given in Table 2.

The Maryland Department of Natural Resources (MD DNR) routinely collects water-quality data in the Potomac River and Potomac Estuary at nine locations, including four stations in TF2 with two each in UTR and LTR, two stations in POTOH, and three stations in POTMH, as shown in Figure 1. Because the areas of the salinity regime-based river segments that have been adopted by the CBP are fairly large, we have also considered smaller river segments around each of the nine water-quality monitoring sites. Each river segment consists of the six-kilometer river-length area which encompasses the water-quality site; that is, the segment comprises the three-kilometer river-length areas immediately above and downstream from a water-quality monitoring site. The SAV coverage for each of these monitoring station segments was determined jointly by workers at the USGS and at the Virginia Institute of Marine Science. SAV coverage in hectares by station segment and year is given in Table 3.

Water-Quality Data
Introduction
Open-File Report 99-219 contents page