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The Coastal Plain of the Chesapeake Bay Watershed

The coastal plain has higher proportions of both cropland (32%) and wetland (21%) than any other physiographic province of the bay watershed. The coastal plain portions of the Chesapeake Bay watershed comprise watersheds with low topographic relief, relatively high moisture infiltration capacities, well-distributed rainfall throughout the year, and unconfined surficial aquifers. Stream flow is mainly derived from groundwater discharge from the surficial aquifer.

Direct or surface runoff in agricultural watersheds generally accounts for about 5 to 15% of Stream flow. The remainder of the precipitation either infiltrates and is available for either groundwater recharge or evapo-transpiration or goes directly into surface water as stream or detention storage. Although this general view of the coastal plain is useful, variations in soils, topography, subsurface stratigraphy, and land use within the coastal plain control the fate of non-point source pollutants relative to the riparian forest buffer systems.

The Chesapeake Bay watershed coastal plain is often divided into inner and outer coastal plains. The inner coastal plain is mostly the western shore of Chesapeake Bay and the upper Eastern Shore. The outer coastal plain is primarily the lower Eastern Shore/Delmarva Peninsula.

Inner coastal plain areas have relatively high topographic relief compared to outer coastal plain systems and generally have finer textured, nutrient-rich soils compared to the nutrient-deficient, sandy soils of the outer coastal plain. A more detailed classification of the coastal plain was developed by the U.S. Geological Survey for the Delmarva Peninsula. This classification of hydrogeomorphic regions was based on qualitative analysis of geologic and geomorphic features, soils, drainage patterns and land cover...

The inner coastal plain includes the portion of the coastal plain located on the western shore of Chesapeake Bay and the area immediately south of the fall line on the Delmarva Peninsula. Tidal sections of rivers extend far into the inner coastal plain, near the fall line in some cases.

Watersheds in the inner coastal plain are characterized by well-drained soils limited to riparian zones. Land use is primarily agricultural on uplands and forested in riparian zones. Topography of this region is gently rolling, with a high degree of stream incision.

The inner coastal plain is a hydrologically complex region because sands and gravels that comprise the surficial aquifer are thin and overlie subcropping sands or finer-textured confining beds of older coastal plain aquifers. Stream valleys are commonly excised into the older units.

As a result of this configuration, the surficial deposits do not form an extensive aquifer as they do in other parts of the coastal plain. Shallow groundwater flow systems in the surficial sediments commonly extend from topographic highs into the deeper aquifer, where they are close to the surface.

In addition, older water from deeper aquifers often discharges upward to streams. If the surficial aquifer overlies a shallow confining bed, groundwater flow is restricted to shallow depths where it comes into contact with riparian zone sediments and soils near aquifer discharge areas.

The Rhode River watershed along the western shore of Maryland is representative of the hydrologic conditions common to much of the inner coastal plain. This 2886-hectare watershed is 62% forest, 23% croplands, 12% pasture, and 3% freshwater swamp. The watershed is underlain by a relatively impermeable clay later, which forms an effective aquiclude. Most groundwater flow to streams is in a shallow surficial aquifer.

The 160-year average rainfall is 108 cm. The long-term average precipitation by season is 28 cm (December to February), 31.4 cm (March to May), 24.5 cm (June to August), and 24.6 cm (September to November).

For the Rhode River watershed, slow Stream flow (base flow or groundwater discharge) averaged 29.6 cm of flow, while quickflow (mostly storm flow or surface runoff from all contributing areas) accounted for 4.97 cm. Studies on Rhode River indicated that 86% of all watershed discharge comes from slow flow or groundwater discharge and 14% from direct surface runoff. For one year of study (1981-1982) it was estimated that about half of all quickflow took place in the summer (June to August) and more than half all slow flow (ground discharge) took place in winter.

(From: Section II: Riparian forest buffer systems in physiographic provinces of the Chesapeake Bay Watershed. In: Water Quality Functions of Riparian Forest Buffer Systems in the Chesapeake Bay Watershed; a report of the Nutrient Subcommittee of the Chesapeake Bay Program, August 1995. Available from the U.S. EPA, Chesapeake Bay Program Office, 410 Severn Avenue, Annapolis, Maryland 21403)