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• PARSONS be a consequence of climatic variability; and the observed rates of erosion represent the net result of some interaction between land use and varying climate. Schumm and Lichty (1963) used historical records and modern channel measurements and observations to evaluate morphologic changes in the channel of the Cimarron River in Kansas during the period 1874 to 1960. During the period 1874 — 1914, the Cimarron River apparently had the planform of a meandering point -bar stream, with a channel averaging approximately 50 feet in width through 6 counties of southwestern Kansas. The floodplain of the river was vegetated, and the morphology of the river in general was relatively stable. Beginning in 1914, and continuing intermittently through 1942, the channel of the Cimarron River widened, until almost all of the floodplain had been removed. The channel widening apparently began during a major flood event (a threshold excursion) that occurred in the spring of 1914. This flood was the greatest of record on the Cimarron River; however, other flood events that occurred between 1914 and 1942 were of sufficient magnitude to further widen the channel, until it reached its greatest average width of about 1,150 feet, and the former floodplain was completely removed. After the flood event of 1942, a reversal of fluvial behavior occurred, and the channel of the Cimarron River began to narrow (Schumm and Lichty, 1963). By 1954 (a period of 12 years after the 1942 flood), the average channel width had decreased to about 550 feet, and the river had replaced about one -half of its floodplain. Conditions of relative stability in morphology had been established by about 1960. The causes of these rapid changes in channel morphology are complex and varied, but are ascribed by Schumm and Lichty (1963) to several primary factors. Once excessive erosion had been initiated by a major flood event, the widening of the Cimarron River channel continued because of the relative ease of bank erosion and the continued occurrence of large floods. Channel widening apparently occurred by bank caving as coarse sand was washed from beneath more resistant sedimentary cover on the floodplain. During channel widening, degradation probably was not an important geomorphic process; however, channel gradient probably was steepened, as the course of the river was shortened by destruction of the meander pattern. Subsequent channel narrowing occurred because the river had widened excessively, precipitation following 1942 was greater than average, and large floods occurred less frequently and did not erode the newly - forming floodplain. The new floodplain is composed of a complex of coalesced islands, abandoned branch channels, and areas of floodplain constructed adjacent to the low -water channel. Large parts of the wide channel were not occupied by low -water flows, and these areas became sites of vegetation growth. The increased plant cover reduced flow velocities over these areas and promoted additional sediment deposition. The authors concluded that floods in semi -arid and and regions may be tremendously destructive to river channels and floodplains (Schumm and Lichty, 1963). This destruction by floods may be a characteristic of erosive processes in drier regions where climatic fluctuations are common, and the streams are ephemeral or carry very low flows during long periods of time. Often these streams cannot adjust as readily as streams in more humid environments to a change in stream regimen or climatic fluctuation. Large floods may trigger an adjustment by initiating periods of severe erosion or deposition, and as a result, large changes in fluvial morphology can occur in relatively short periods of time. SAES \WP\PR0JECTS\3- StatesWl Final Tech Metno.doc -11-