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<br />and destroyed agricultural lands, irrigation stuctures, roads, <br />railroads, bridges, and buildings. <br /> <br />The 1884 flood is considered the most severe flood known in Monte <br />Vista; the estimated peak discharge was 20,000 cubic feet per second <br />(cfs) at Alamosa, Colorado. <br /> <br />In June 1927, a peak flow of 18,500 cfs was recorded in Monte Vista. <br />Flooding was caused by 4 days of cloudbursts and incessant rain. <br /> <br />The October 1911 flood is considered the most severe flood known <br />in Del Norte. The Rio Grande inundated the railroad tracks just <br />outside the depot. Water was also 1.5 to 2 feet deep at the inter- <br />section of 4th and Pine Streets. The flood was triggered by in- <br />tense rains in the mountains over an extended period of time. <br />Diversion ditches, canals, and arroyos carried almost twice as <br />much water as compared with other years. The U.S. Geological Sur- <br />vey (USGS) has estimated the flow to have been 18,000 cfs (Refer- <br />ence 4). This represents a recurrence interval of greater than <br />500 years. <br /> <br />The highest flood of record on the South Fork Rio Grande was that <br />of October 1911. It destroyed four bridges, including the Denver <br />& Rio Grande Western Railroad bridge near the mouth of the river. <br />The gage was destroyed, but subsequent investigation found that <br />the river began to rise at about 7:00 a.m. on October 5, and rose <br />at a rate of approximately 1 foot per hour, overflowing its banks <br />at approximately 11:00 a.m. It stayed at the highest stage until <br />approximately 2:30 p.m. At 7:00 p.m., the river was again within <br />its banks. From high-water marks, the peak discharge on the South <br />Fork Rio Grande was estimated to be approximately 8,000 cfs. This <br />flood discharge was greater than the discharge expected from a <br />100-year recurrence interval flood. The flood of June 1927, the <br />highest since 1911, reached a stage approximately 1 foot lower <br />than the flood of 1911, but no estimate of the discharge is avail- <br />able for the South Fork Rio Grande (References 1, 2, and 3). <br /> <br />2.4 Flood Protection Measures <br /> <br />The Rio Grande Reservoir Dam was raised in 1979, with modifications <br />made to the emergency spillway. The spillway capacity is 10,000 <br />cfs. The estimated 100-year peak inflow is 5,600 cfs, which indi- <br />cates that the 100-year reCurrence interval flood will not overtop <br />the dam. <br /> <br />Other than the dam, no other structural flood protection measures <br />are known to exist in Rio Grande County. <br /> <br />6 <br />