Geology and Geomorphology
The Big River basin (Figure Ge01) lies within two subdivisions (Salem Plateau and St. Francois Mountains) of the Ozark Plateau physiographic region (MDNR 1986). Land elevations range from 435 feet above sea level at the mouth of Big River to 1,740 feet in the headwaters at Buford Mountain.
The Big River basin contains geologic formations (Figure Ge02) ranging in age from Mississippian to Precambrian. The majority of basin streams flow through the Salem Plateau, which is a dissected plateau of sedimentary rock topped by a thin layer of glacial loess. This plateau commonly forms rolling to narrowly-cut river valleys. As Big River flows northward, it cuts through progressively younger limestone and dolomite. Despite Karst topography being locally prominent, few springs are present. Sandstone is common in Jefferson County and shale becomes prominent in the lower basin.
Substantial deposits of lead, zinc, copper, magnesium, and barite have attracted mining operations to Jefferson, St. Francois, and Washington counties beginning over 200 years ago (MDNR 1984).
The southeastern portion of the basin drains the northern edge of the St. Francois Mountains which feature rugged, igneous peaks thought to be unaltered from their time of creation. Since these formations are highly-resistant to erosion, streams tend to be high gradient and form very narrow river valleys through thin residuum.
Discussion of soil types is limited due to unavailability of soil surveys for Jefferson and Washington counties (68% of watershed). Primary soil series in the upper watershed include: Crider, Fourche, and Hildebrecht on ridge tops; Gasconade, Goss, and Irondale on slopes; and Haymond and Midco in the bottoms (USDA 1981, 1985, 1989, 1991). Soils on ridgetops and slopes are highly erodible, especially when disturbed.
Upper basin soils are typical for the Ozark Dome region, while lower basin soils reflect the Ozark border region (MDNR 1986). Upland soils are moderately shallow and consist of a combination of loess and residuum derived from in-place weathering of dolomite. These soils are silty, moderately well drained, highly susceptible to erosion, and suitable for pasture, forest, and limited row cropping (USDA 1981, 1985, 1989, 1991). However, much of the loess and residuum has been eroded from the slopes, exposing much chert and frequent bedrock outcrops.
The lower elevations of these soils tend to be clayey with high chert content, thin, draughty, infertile, and stony, best suited for grasslands and forest (USDA 1981, 1985, 1989, 1991). Very fertile silt-loam, developed from alluvium, has been deposited over cherty gravel in river valley bottoms and is suitable for row crops, bottomland forest, and pasture.
Big River becomes a sixth order stream at its confluence with Cedar Creek at river mile (RM) 118 in Washington County. A total of 86 streams (> order 3) were identified, ordered, and measured (Table Ge01) from USGS 7.5-minute topographic maps (Table Ge02).
There are 129 miles of permanent streams and 220 miles of intermittent streams in the basin (Funk 1968).
The Big River watershed encompasses 955 square miles. Main sub-basins (Order 5) range from 26 to 189 square miles, with the largest being Mineral Fork (Table Ge03).
Big River's average gradient is 6.6 ft/mile and ranges from 1.3 (RM 34) to 200 (RM 137). Generally, stream gradient is steepest in the St. Francois Mountain area and more gradual beginning at RM 85 to the confluence with the Meramec River.