CLIMATE, POPULATION, AND LAND-USE 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 deg. F. (Flora, 1948:148). According to Kincer (1941:704-705) the average temperature in July, the warmest month, is 78 deg. F., and the coolest month, January, averages 28 deg. 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.