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<br />On May 8 and 9, 1957, approximately 3 to 5 inches of rain fell over <br />the entire basin of St. Vrain Creek. The rain began at <br />approximately 10 p.m. and stopped at approximately 6 a.m. On May 9, <br />at approximately 1 a.m., St. Vrain Creek peaked at Lyons at 3,060 <br />cfs. The flood damaged and destroyed irrigation diversion works <br />and bridges downstream from Lyons. <br /> <br />In 1969, heavy rainfall combined with snowmelt caused prolonged <br />high flows on St. Vrain Creek. The worst flooding occurred on <br />May 7 and 8 and from June 15 to June 21. Roads and bridges along <br />the stream were extensively damaged, streambanks were eroded, and <br />farmlands were flooded. On May 7, the peak discharge at Lyons was <br />2,900 cfs. (Photographs of the 1969 flood are shown on Figures 17, <br />18, 19, and 20.) <br /> <br />The steep stream slopes create swift currents during a flood, which <br />produces added damages. Debris carried by the fast-moving water <br />not only threatens bridges and culverts, but batters houses or <br />other structures on the floodplain. The bridge and culvert <br />crossings often result in channel restriction, raising the water- <br />surface profile. Erosion undercuts and destroys structures which <br />would otherwise receive little damage from inundation. Large <br />quantities of rocks are often deposited in portions of the channel, <br />leaving little capacity for future floods. Vegetative growth in <br />the channel also reduces the channel capacity and causes increased <br />overbank flow during high floods. Major flood damages in the study <br />area are to roads, culverts, and residential buildings. <br /> <br />Early records of floods in the Town of Nederland study area are <br />fragmented and lacking in detail. A description of the more-recent <br />floods follows: <br /> <br />June 22, 1951 - Boulder Dailv Camera: <br /> <br />Nederland reservoir at the head of Boulder Canyon is full <br />for the first time in two years. <br /> <br />With water pouring over the l32-foot-high spillway at the <br />dam and heavy rains falling in the mountains, a flood <br />threat developed in Boulder Creek Thursday night but there <br />was little damage. Contrary to broadcast reports, the <br />water did not reach the canyon highway or cause any <br />interference with road bridges...It is not unusual for the <br />reservoir to fill; it has happened more than half of the <br />years since the dam was completed in 19l0. But, with the <br />dam unable to hold any more water, it means a possible <br />flooding of Boulder Creek if there are heavy rains and <br />fast thaws of snow at the headwaters. <br /> <br />27 <br />