Analysis of Historical Distribution of Submerged Aquatic Vegetation (SAV) in the York and Rappahannock Rivers as Evidence of Historical Water Quality Conditions

Historical black and white format photographs at scales of approximately 1:20,000, dating from 1952 to 1956 were used to delineate the maximum coverage of SAV in the study region. Coverage of photography from decades before and after this period were found to generally to be of poorer quality and show less SAV presence. Photo-interpretation of the aerial photographs was accomplished using a head-up, on-screen digitizing system at fixed image scale of 1:12,000 and followed as closely as possible the methods currently used to delineate SAV beds throughout the Chesapeake Bay as well as the delineation of historical SAV coverage for other region. A total of 16,340 hectares of sub-tidal bottom in the western shore bay region between the James and Potomac Rivers including all of the York and Rappahannock Rivers were found to display SAV signatures. Of this total approximately 11,260 ha, or 69%, were determined to be growing at depths shallower than 1 m MLW (Mean Low Water), 4,200 ha or 26% between 1 m and 2 m MLW, and 884 ha or 5% at depths below 2 m MLW. Comparison of the historical depths of growth with that of photography taken in 1999 reveal a general decrease in maximum depth of growth of approximately 0.5m in many areas. The most upriver areas of the York and Rappahannock, where SAV no longer are found, had SAV bed signatures to 1 m MLW, while downriver areas and regions along the Chesapeake Bay had maximum depths or 2 m or more in some areas. Losses of vegetation have been much more extensive in the Rappahannock than the York. In 1999, in the lower York River (YRKPH) approximately 23% of the historical SAV coverage remained while only 1% remained in the lower Rappahannock (RPPMH). Areas along the bay shoreline had the highest proportion of remaining beds with bay segments CB6PH and MOBPH exceeding 50% of historical coverage