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Beautiful Cool Clear Mountain Streams Carry the Snowmelt to the Great Plains flashy on all except the major streams of the area. Rain- fall is the primary cause of floods with snowmelt and ice jams being infrequent and limited factors in flood occurrence. The Great Plains are noted for their rather scanty and erratic annual precipitation. Average annual precipitation on the plains varies from lows under] 2 inches to highs slightly over 20 inches. As a generalization, the highest annual precipitation in the plains portion of the basin occurs in the southeast and decreases steadily as one moves to the northwest. Rainfall averages are a bit misleading for individual yearly precipitation tends to vary quite widely. The plains are well known for their noncyclic and often severe droughts. These droughts can last from a single summer to several years in duration, and there is no known means by which to accurately predict their occurrence. The drought is the most severe climatic hazard faced by the basin residents. Despite the low average annual precipitation, agricul- ture is possible on the Great Plains in part because of the seasonal variation in the rainfall. The greatest amounts of rainfall come during the spring months of May, June, and early July in most years. Occasionally, the plains experience high intensity thunder storms during the summer, often accompanied by high winds and hail. These storms commonly cause more crop damages than benefits from the rain they produce. 10 Winter snowfall is generally light in the plains with January and February being the driest months of the year. The Great Plains have low annual runoffs, generally less than one inch. Groundwater flow in most streams is very limited. As a result, the typical plains' stream has a high flow during the spring rains but tends to dry up during the summer, with occasional short-duration high flows from passing thunder showers. A distinct contrast to the general condition is found in the Loup Rivers which flow out of the Sand Hills of north-central Nebraska. The flow of these rivers is sup- plied, in most part, by a relatively steady discharge of groundwater due to the large area intake and the high porosity of the thick sands covering the area. Flood flows in the plains are semi-flashy in most streams but the rolling to steep topography of some localities produces typical flash floods. Floods in the northern part are caused by either rainfall or combined rainfall and snowmelt, with ice jams being involved fre- quently in the early spring floods. In the southern part of the plains, rainfall is the primary cause of floods and neither snowmelt nor ice jams are important factors. The Central Lowlands are generally well endowed \vith annual precipitation \vit..~ the annual averages ranging from 20 to 40 inches. Yearly precipitation in the northern portion is divided between summer rains and winter snow while in the southern portion, annual pre- cipitation is composed almost entirely of rainfall which occurs throughout the year.