Climate of the Big Blue River Basin is of the subhumid continental type, with an average annual precipitation of 22 inches in the northwest and 30 inches in the southeast. The mean annual evaporation from water surfaces exceeds annual precipitation by approximately 30 inches (Colby, et al., 1956:32-33).
The average annual temperature for the basin is 53° F. (Flora, 1948:148). According to Kincer (1941:704-705) the average temperature in July, the warmest month, is 78° F., and the coolest month, January, averages 28° F. Periods of extreme cold and heat are sometimes of long duration. Length of the growing season varies from less than 160 days in the northwest to 180 days in the southeast (Kincer, loc. cit.).
The human population of the Big Blue Basin varies from about 90 persons per square mile in one Nebraska county in the northwest and one Kansas county in the southeast, to as few as six persons per square mile in some northeastern counties. The population is most dense along the eastern border of the basin, decreasing toward the west. This decrease in population is correlated with the decrease in average annual precipitation from east to west (Colby, et al., 1956:80).
The principal land-use in the Big Blue Watershed is tilled crops, with wheat, sorghums, and corn being most important. Beef cattle are important in some portions of the basin. Colby, et al. (1956:24) reported that in 1954 as much as 55 per cent of the land in some counties near the mouth of the Big Blue River was in pasture. Only one Nebraska county had less than 15 per cent in pastureland.
PHYSICAL FEATURES OF STREAMS
Streams of the Big Blue River Basin are of three kinds: turbid, sandy-bottomed streams, usually 150 to 300 feet in width; relatively clear, mud-bottomed streams, ten to 60 feet in width; and clear, deeply incised, gravel-bottomed streams, usually five to 30 feet in width.
Sand-bottomed Streams.—The Big Blue and Little Blue rivers represent this kind of stream. The bottoms of these rivers consist almost entirely of fine sand; nevertheless, their channels are primarily deep and fairly uniform in width, rather than broad, shallow, and braided as in the larger Kansas and Arkansas rivers in Kansas ([Plate 11, Fig. 1]). In the Big Blue River, gravel occurs rarely on riffles, and gravel-rubble bottoms are found below dams ([Plate 11, Fig. 2]). The Big Blue flows over a larger proportion of gravelly bottom than does the Little Blue.
Big Blue River rises at about 1,800 feet above mean sea level and joins the Kansas River at an elevation of 1,000 feet above m. s. l. The average gradient is 2.8 feet per mile. Little Blue River, originating at 2,200 feet, has an average gradient of 5.3 feet per mile, entering the Big Blue at 1,100 feet above mean sea level (Nebraska State Planning Board, 1936:628, 637). The Little Blue is the shallower stream, possibly because of the greater amount of sandy glacial deposits in its watershed and the swift flow that may cause lateral cutting, increased movement, and "drifting" of the sandy bottom.
For approximately a 50-year period, stream-flow in the Big Blue River at its point of entry into Kansas (Barnston, Nebraska) averaged 603 cubic feet per second, with maximum and minimum instantaneous flows of 57,700 c. f. s. and one c. f. s. The Little Blue River at Waterville, Kansas, averaged a daily discharge of 601 c. f. s. (maximum 50,400, minimum 28). Below the confluence of the Big Blue and Little Blue rivers, at Randolph, Kansas, the average daily discharge was 1,690 c.f.s. (maximum 98,000, minimum 31) (Kansas Water Resources Fact-finding and Research Committee, 1955:27).
The turbidity of the Big Blue River, as determined by use of a Jackson turbidimeter, varied from 27 parts per million in winter (January 10, 1958) to as high as 14,000 p.p.m. (July 12, 1958). The Little Blue River has similar turbidities, with high readings being frequent. In the summer of 1957, pH ranged from 7.2 to 8.4 in the Big Blue River Basin—values that correspond closely with those of Canfield and Wiebe (1931:3) who made 25 determinations ranging from 7.3 to 8.3 in the streams of the Nebraskan portion of this basin in July, 1930. Surface temperatures at various stations varied from 38° F. on January 10, 1958, to 90° F. in backwater-areas on July 19, 1957. The average surface temperature at mid-day in July and August, 1957, was approximately 86.5° F.