Fluvial sediment and chemical quality of water in the Little Blue River basin, Nebraska and Kansas

The Little Blue River drains about 3,37)0 square miles in south-central Nebraska and north-central Kansas. The uppermost bedrock in the basin is limestone and shale of Permian age and sandstone, shale, and limestone of Cretaceous age. Bedrock is exposed in many places in the lower one-third of the basin but elsewhere is buried beneath a thin to thick mantle of younger sediments, mostly of Quaternary age. These younger sediments are largely fluvial and eolian deposits but also include some glacial till. Consisting in large part of sand and gravel, the fluvial deposits are an important source of ground-water supplies throughout much of the upper two-thirds of the basin. Loess, an eolian deposit of clayey silt, is by far the most widespread surficial deposit. The climate is continental. Temperatures ranging from -38 ? F to 118 ? F have been recorded in the basin. Average annual precipitation as low as 10.31 and as high as 49.32 inches has been recorded. During most years in the period 1956-62, when nearly all the water-quality data were obtained, annual precipitation and annual runoff were greater than normal. Flow-duration data indicate, however, that the flow distribution for the period was near normal. The Little Blue River has the same suspended-sediment characteristics as nearly all unregulated streams in the Great Plains--a wide range in concentrations, low concentrations during low-flow periods, and high concentrations during almost all periods of significant overland runoff. The maximum instantaneous concentration normally occurs many hours before maximum water discharge during any given rise in stage; the maximum daily mean concentration during any given year normally occurs at a moderate stream stage, not during a major flood.

Suspended-sediment data for Little Blue River near Deweese, Nebr., which receives drainage from the upstream third of the basin, approximately, show that during the 1!}57-61 water years concentrations of 100 ppm (parts per million) or less prevailed about 42 percent of the time and concentrations of 1,000 ppm or less prevailed about 85 percent of the time. Observed concentrations ranged from 2 to 21,000 ppm: daily mean concentrations ranged from 2 to 13,800 ppm.

The discharge-weighted suspended-sediment concentration was computed as about 2,800 ppm at Little Blue River near Deweese, about 3,300 ppm near Fairbury (Endicott), and about 3,000 ppm at Waterville. These stations receive drainage from about one-third, two-thirds, and nearly all the basin, respectively. Water-utilization problems resulting from high concentrations are not significant in the basin ; use of water from the Little Blue River is quantitatively negligible. Concentrations and, consequently, discharges of sediment are greater at a given water discharge on a rising stage than at the same discharge on the falling stage of the same runoff event. Also, a wide range in sediment discharge occurs at similar water discharges during different runoff events. Daily sediment discharges at Little Blue River near Deweese ranged from about 1,400 to 16,000 tons at daily mean water discharges of about 500 cfs (cubic feet per second) and from almost 7,500 to 28,000 tons at water discharges of about 1,000 cfs.

The estimated long-term sediment discharge at Little Blue River near Deweese is about 400,000 tons per year: near Fairbury, about 1,200,000 tons per year: and at Waterville, about 1.900,000 tons per year. The high sediment discharge from the downstream part of the basin is due to greater precipitation and runoff--not to higher concentrations of suspended sediment--in the downstream parts of the basin.

Nearly all the suspended sediment is silt and clay. The streambed material is mainly medium sand to gravel. The median particle size of bed material observed was about 0.73 mm near Deweese and about 0.77 mm near Fairbury. A few computations of total sediment discharge of Little Blue River near Deweese indicate that suspended-sedim