Geology and Geomorphology
The Fabius River basin lies in the eastern section of the Glaciated Plains (Figure Ge01) Division of Missouri (Thom and Wilson 1980), also known as the Dissected Till Plains. The Till Plains were formed by glaciers that deposited drift composed mostly of clay with some rock, gravel and sand lenses (MDNR unpublished). Geologically (Figure Ge02), the basin changes significantly from northwest to southeast. Glacial till up to 200 feet thick on ridgetops is found in the upper portions of the basin, mainly the upper North and Middle Fabius sub-basins. It thins only slightly on gentle slopes and in broad valleys. Four to eight feet of wind-deposited loess overlies this till. Beneath it is a thin layer of sand and gravel and then a layer up to 400 feet thick of alternating deposits of Pennsylvanian age sandstone, siltstone, shale, limestone, and coal.
In the middle and lower portions of the basin the topography grades from broad plains to steep, abrupt valleys with high relief. Till shallows quickly on the lower slopes to expose Mississippian age limestone in the valley walls and streambeds. Loess deposits are usually less than four feet deep in lower North and Middle Fabius sub-basins and in the South Fabius drainage. This region of thin glacial soils and exposed limestone is roughly defined as the area downstream of Route E in Lewis County in the North Fabius sub-basin, downstream of the Scotland-Knox county line in the Middle Fabius drainage, and downstream of Edina, Missouri in the South Fabius sub-basin. The basin flattens as it enters the Mississippi River floodplain, and the substratum turns to fine alluvium.
The majority of the basin is located in the Central Claypan region (Allgood and Persinger 1980). Soils of this region are formed in glacial till or loess parent material or both (SCS 1992b, 1984, 1979, 1975). They generally have a silt loam surface of moderate to high erosion potential overlying a silty clay subsoil of low permeability. Once home to native prairie grasses, most of this fertile region is now considered excellent farmland. Deep loess soils occur in the upper North and Middle Fabius drainages, and soils of the Central Mississippi Valley Wooded Slopes are found on steep hills and some ridgetops primarily in the lower part of the basin. Silty loam alluvial soils are limited to stream floodplains. Due to the clay content of the till and the underlying shales and limestone, vertical movement of water from the surface to groundwater is minimal throughout the basin (MDNR unpublished). No significant springs exist so stream flow is largely dependent on surface runoff.
Stream Channel Gradients
Channel gradients (slopes) were determined for all third-order and larger streams by using USGS 7.5-minute topographic maps and digitizing software (Table Ge01). Gradient is very low in the lowermost reaches of the Fabius and North Fabius rivers (2.0-2.8 feet/mile). Gradients in fifth-order reaches of basin streams range from 2.6 feet/mile in the Middle Fabius River to 5.0 feet/mile in the south forks of both the Middle Fabius and South Fabius rivers. Because of their higher gradients, the latter two streams exhibit better riffle/pool development than many lower-gradient prairie streams of similar size. Gradients in fourth-order reaches of basin streams range from 3.9 to 11.5 feet/mile.
While third-order reaches of basin streams have wide-ranging gradients (Table Ge01), the slopes of some short, third-order streams are strikingly high. For instance, gradient exceeds 90 feet/mile in an unnamed tributary in the lower portion of the North Fabius River sub-basin. This and other high-gradient streams are generally located in the middle and lower portions of the basin as the watershed enters the region of steep, narrow valleys with shallow till and exposed limestone.