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Corps of Engineers by the National Weather Service and is developed from major storms that have occured in the region. , In the 1940's, local interests constructed a levee system along the Rio Grande to protect Alamosa. The levee system begins just north of Alamosa and continues to the south side of the community. Since the construction of these levees, their structural integrity has been reduced to the point where they are essentially no more than spoil banks. Trees growing on the levees have further reduced their effectiveness by causing piping through the levees during high flows. Residential development in the northwest part of Alamosa and East Alamosa also has contributed to the increasing flood problem. Flows equal to or greater than the 10-year event (4,600 cfs) will outflank the existing levee system and cause damage to Alamosa and East Alamosa. .... B. Historical Floods: The earliest recorded flood was in 1869. Little is known about this flood other than some damage did occur in Alamosa. A flood in 1884 was caused by the melting of a heavy snow cover augmented by heavy rains. The flood of 1884 was caused by the melting of a very heavy snow cover. This flood peaked on June 14th and 15th, and was probably the flood of record. The flood of 1905 was the only destructive flood in which rainfall as not a factor. The flood of 1911 was the only one recorded in the upper Rio Grande Basin that was caused wholly by heavy rains. The flood of 1927 was the highest since stream gage records have been kept. This flood was caused by melting snow and increased by precipitation. More recently, high flows occured in 1979, 1985, and 1986 in the Alamosa area. Only a successful flood fighting effort in all three years prevented substantial damage within the study area. V. EXPECTED FUTURE WITHOUT CONDITION Without the implementation of flood control measures, the communities of Alamosa and East Alamosa will remain suseptable to a severe flood threat from high flows of the Rio Grande. Existing average annual damage in the study area are currently estimated at $1,093,000. Assuming that no new residences or businesses moved into the 100-year floodplain, damages and interruption of services will continue in the future. Without a flood fighting effort these damages will begin to occur between the five and ten year event. The existing levee system, while providing minimal protection, will continue to deteriorate, and provide even less protection. VI. FLOODPLAIN INVENTORY A. Area of Consideration: The flood plain of the Rio Grande in the vicinity of Alamosa, Colorado, encompasses approximately 5,000 acres. For the purpose of this analysis, the flood plain divided into two economic reaches. Reach 1: Locted on the South Bank of the Rio Grande, Reach 1 is urbanized and includes the downtown Alamosa commercial area. Reach 2: Location on the North Bank characterized by low-to-moderate density of the Rio residential Grande, it is area with some 7