Watershed Landuse and Bay Sedimentation

Abstract

This paper discusses a unique synthesis of techniques used to examine sediment dynamics in a temperate watershed estuary system. Sedimentation rates for Tomales Bay, California, were initially calculated using a geographi8c information system (GIS) and digitized bathymetric data from National Ocean Service navigational survey charts published in 1861, 1931, 1957 and 1994, with a public-domain Unix-based software package, Generic Mapping Tool (GMT). The surfaces were subtracted from each other to determine the volume of sediment that accumulated in the Bay between surveys. Average baywide accumulation rates, normalized by watershed area, were found to be about 94 t km^-2 yr^-1 for the 1861-1931 interval (Interval I), 357 t km^-2 yr^-1 for 1931-1957 (Interval II), and 101 t km^-2 yr^-1 for 1957-1994 (Interval III). GIS-derived rates for Tomales Bay were compared with results hindcast from rating curve equations between rainfall and contemporaneous sediment yield. GIS results for Intervals I and II agree rather well with the sediment yield equation, while the rating curve provides a much lower sediment delivery rate than the bathymetric data for Interval II. Differences between the two sets of rates apparently reflect changes in land use and streambed sediment storage.