The watershed lies in two physiographic provinces, the Piedmont to the north and the Atlantic Coastal Plain to the south, which are separated by the Fall Zone. The Fall Zone consists of rapids and falls that form a natural barrier to transportation on streams that flow into the Chesapeake Bay.

The topography of the Gwynns Falls watershed is controlled largely by fluvial erosion associated with the temperate and humid climate of the mid-Atlantic coast. This causes the topography to vary from "gently sloping" to "hilly" with locally steep slopes and bedrock outcroppings within drainage corridors (Survey 1929; Froelich and others 1980).

The vegetation of the watershed has changed from primarily forest before it was settled by Europeans to primarily herbaceous today. There are no original stands of forest in the Baltimore area. After European settlement, farmers cultivated most of the land. A report by the State Forester in 1929 indicates that only 29 percent of the County was forested, and that most of these forested areas were in lots of 4.1 to 40.5 hectares. These forested areas were located primarily on steep slopes and non-arable lands and held almost exclusively by farmers. Hardwoods (96%) were pre-dominant in the forests of Baltimore County in 1929 and consisted mainly of three basic forest types: a ridge type, primarily chestnut oak (Quercus prinus) and scarlet oak (Quercus coccinea); a slope type, with scarlet oak, black oak (Quercus velutina) and white oak (Quercus alba) transitioning to red oak (Quercus rubra), tulip poplar (Liriodendron tulipifera) and hickory (Carya ovata) on the lower slopes; and a bottom type consisting mainly of red maple (Acer rubrum), ash (Fraxinus pennsylvanica), elm (Ulmus americana), birch (Betula nigra) and sycamore (Platanus occidentalis) (Survey 1929:395).

A study of the forests of Maryland (Brush 1980) shows that forest stands in the Gwynns Falls watershed include the chestnut oak association in the upper watershed which is underlain by coarser soils weathered from schist, the tulip poplar association in the lower part of the watershed which occurs on thicker saprolitic soils weathered from gneiss and granite, and the box elder-green ash-sycamore-silver maple association in the riparian areas. An outcrop of serpentine just outside of the upper watershed supports the blackjack oak-post oak-chestnut oak association. Virginia pine (Pinus virginiana) and red cedar (Juniperus virginiana) are the only coniferous species in the watershed. American chestnut (Castanea dentata) was a dominant tree in this area prior to its extinction in the 1920s to 1930s, due to the chestnut blight. Small chestnut stems continue to rise from root stumps in forest stands, and continue to be killed by the disease. Chestnut has been replaced throughout the forest by neighboring species.

A 1994 forest survey of Baltimore County and City by the Maryland Forest Service estimated that coniferous forests (1.9%) and deciduous forests (19.4%) constituted only 21.3% of the Baltimore City and County area. Landsat TM data indicates that in 1992 only 18.9% of the Gwynns Falls watershed was forested.

Rainfall and runoff is generally uniform throughout the year. Average annual precipitation is about 109 cm./yr. and stream discharge is approximately 38 cm./yr. Maximum evapotranspiration occurs during July, and groundwater reservoirs are recharged primarily between mid-September and March (Froelich and others 1980). The greatest rainfall intensities occur in the summer and early fall and precipitation from this period is about 10% higher than during the remaining three seasons of the year. Summer is also the season for convectional storms and hurricanes and precipitation during these types of storms are typically of high intensity and short duration over a relatively limited area (Black 1991). Severe droughts are rare (Survey 1929). The proximity of large bodies of water and the inflow of southerly winds contribute to relatively high humidities during much of the year.