Average annual precipitation for the basin ranges from 31 inches in the northern part of the basin to 35 inches in the southern portion of the basin (USDA-SCS 1982). The greatest amount of precipitation usually occurs during June, while the least amount of precipitation generally occurs during the months of January and February (Figure Hy01). The basin is covered primarily in glacial till, and due to the clay content, movement of water to the subsurface is greatly diminished (Detroy and Skelton 1983). Thus, most rainfall runs off the surface rather than percolating into the soil. Streams in the basin show rapid flow increases in conjunction with rains, but quickly return to low flow conditions shortly after runoff ceases (MDNR 1995). Average annual runoff ranges from five inches in the northern portion of the basin to seven inches in the southern part of the basin (USDA-SCS 1982).
United States Geological Survey (USGS) Gage Stations
There are five active and 26 inactive USGS water gage stations throughout the Platte River basin (Table Hy01). Two of the active gage stations are located on the 102 River in Iowa, near the towns of Gravity (crest station) and Bedford (Figure Hy02). The remaining three active stations are located in Missouri. One station is located on Little Platte River 2.4 miles below Smithville Reservoir, one is located at Smithville Reservoir in the dam, and the remaining one is located on the Platte River at Sharps Station (Figure Hy02). Discharges at these active gaging stations are presented in Table Hy02.
Permanency of flow in streams within the Platte River basin is best illustrated on USGS 7.5 minute topographic maps. The USGS identified perennial reaches of streams (defined as having water 12 months of the year during years of normal precipitation) on these maps with solid blue lines. Intermittent streams (defined as having water less than 12 months of the year) were indicated with broken blue lines. A listing of the 7.5 minute quadrangle maps covering each of the 435 third order and larger streams within the Platte River basin is provided in Table Hy03 (Figure Hy03).
Funk (1968) classified Missouri streams as permanent if they maintained flow during drought. Intermittent streams were defined as those that maintained permanent pools when flow ceased during drought periods. Based on these criteria, Funk (1968) determined that the entire 138 miles of the Platte River in Missouri had permanent flow. In addition, the entire 70 miles of the 102 River in Missouri maintained permanent flow as well (Table Hy04). Information on the permanence / intermittence of flows in smaller tributaries within the basin is presented in Table Hy04.
Average annual discharge for the Platte River at Sharps Station, which is 3.3 miles below the confluence of the Little Platte River in Platte County at river mile 25.1 (98% of the drainage basin), is 1,925 cfs (Table Hy02). Examination of the flow duration curve for the Platte River at Sharps Station shows that temporal discharge variability within the basin is high (Figure Hy04). The highest instantaneous daily flow was 37,800 cfs recorded on July 26, 1993, while the lowest instantaneous daily flow was 12 cfs recorded on August 7, 8, 13, and 14, 1989. Stream flows within the basin are generally lowest in January, while peak flows occur during May, June, and July (Figure Hy05), and these flows coincide with monthly precipitation values. Low flows in the Little Platte River below Smithville Dam are maintained at 8 cfs. Hauth (1974) presented flood magnitudes at various recurrence intervals for several locations within the Platte River basin, and these are presented in Table Hy05.
Skelton (1976) noted that low-flow characteristics of streams vary among physiographic regions in Missouri, and that the low-flow potential of most streams in the Dissected Till Plains region is poor because of the low hydraulic conductivity of the clays and shales in the area. Skelton (1976) estimated that the 7-day Q2 would be zero for drainage basins less than 100 miles2. In addition, about 60% of streams with drainage basins of 100 to 200 miles2 would have 7-day Q2 values of zero, and the remaining streams of this size would have 7-day Q2 values ranging from 0.1 to 1.0 cfs. Skelton (1976) also estimated that the 7-day Q10 would be zero for drainage basins less than 200 miles2 in this region, with about 70% of the streams with drainage basins of 200 to 1,000 miles2 having 7-day Q10 values of zero. The remaining 30% would have 7-day Q10 values ranging from 0.1 to 1.5 cfs. Data from the Platte River basin for seven day low-flows at two and ten year intervals were reported in Skelton (1970) and Skelton (1976), and these are presented in Table Hy06. The slope index (the ratio of the 7-day Q2 to 7-day Q20) for the Platte River at Agency was 73.3, and this high value indicates extremely high variability in annual low flows and poor groundwater supply. The average slope index for ten streams within the Dissected Till Plains region was 25 (Todd et al. 1994), and ranged from 8.7 in the Grand River near Gallatin to 73.3 for the Platte River. Channelization and watershed modifications were attributed to the wide range in slope index values within northern Missouri (Todd et al. 1994).
Dam and Hydropower Influences
Smithville Lake, a 7,190-acre impoundment on the Little Platte River, and Mozingo Lake, a 1,000-acre impoundment on Mozingo Creek, are the two largest impoundments within the basin. In 1984, there were 59 impoundments greater than two surface acres within the Missouri portion of the basin, and these totaled 635 acres (MDNR 1995). Although no information exists on the total number of impoundments within the Iowa portion of the basin, ten public impoundments totaling 1,042 acres were identified. Undoubtedly, the number of impoundments greater than two acres has increased dramatically throughout the basin since 1984 in association with PL 83-566, SALT, EARTH, and other erosion control projects. Concern exists on what effects these impoundments have on low flow conditions because they intercept runoff and provide little or no provisions for maintenance of stream flows. There are no hydropower facilities within the Platte River basin.