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ARKANSAS RIVER BASIN, COLORADO, KANSAS, NEW MEXICO D7 probably exceeded the 50-year flood at many of these sites. By this com- parison, the 1965 floods were obviously outstanding. CAUSES Flooding is often caused by a single event; the ensuing damage, by the downstream progression of the flood wave. For the June 1965 floods, es- pecially on the main river courses, a series of flood waves caused progressive flooding. Colorado, in general, received fairly heavy rain on June 14; hail damaged some areas, particularly around Colorado Springs. Rainfall was light on June 15, torrential on June 16, and of tremendous proportions on June 17. New Mexico received intense rainfall in the Mora River basin on June 15 and in the upper Canadian River basin on June 17. The June 16 and 17 storms covered practically all areas in southeastern Colorado and northeastern New Mexico, with greatest concentration of rainfall near Holly, Lamar, and Two Buttes, Colo. ANTECEDENT CONDITIONS General rains began in the Arkansas River basin in Colorado on May 22, and as much as 2.68 inches of rain was reported for a single day. June 4 and 5 were also days of relatively high precipitation, and the stage was set for "rivers to roll" beginning June 14. General rains in New Mexico June 9-12 set the stage for the beginning of high runoff in the Canadian River basin. PRECIPITATION The U,S, Weather Bureau (1961) has developed maps showing relations between rainfall, intensity, and frequency. These relations are only general and do not reflect the orographic effect of relatively isolated topographic features. The orographic effects of the divide between Colorado Springs and Limon, Colo., and the divide extending from a point between Trinidad, Colo., and Raton, N. Mex" generally eastward to the Panhandle of Oklahoma, were quite pronounced during the storms of June 16 and 17. Thus, a direct comparison between regional rainfall relations and some of the observations of point rainfall could be misleading. However, if the anomalies are considered, the comparison should provide some perspective for an evaluation of the rainfall that did occur. From the regional relations, the 100-year 6-hour rainfall in the vicinity of Fountain, Colo., is 3.3 inches; the observed 6-hour rainfall at Fountain on June 17 between 1100 and 1700 hours (astronomical time) was 4.70 inches. The lOO-year 24-hour rainfall in the vicinity of Holly, Colo., is 5.2 inches; the observed 24-hour rainfall at Holly on June 17 was 11.08 inches. The U,S. Army Corps of Engineers (1966) reported that in the 14 hours before 1400 hours June 16, 15.5 inches of rain was observed 28 miles south- southeast of Lamar, Colo. This is about three times the lOO-year 12-hour r~;nf~1I