regions, and most of the Arkansas-White-Red and 

 the Great Basin regions. 



Major efforts have been made — by construction 

 of reservoirs and channel dredging — to maintain 

 instream flow levels that are sufficient for commercial 

 navigation on the inland waterways system. While 

 mitigating the effects of variable rainfall on the 

 acreage level of instream flows, these efforts have 

 greatly modified aquatic habitat conditions. Water 

 depths and movement have been changed on long 

 stretches of many streams. At the same time, new 

 habitat has been created in reservoirs. 



The effect of forests and other vegetation on runoff 

 and streamflows, especially in reducing wide varia- 

 tions in flow, has long been known. Increasing atten- 

 tion is being directed at nonstructural methods, in- 

 cluding vegetation management, as alternatives to 

 dams and channelization for minimizing wide swings 

 in streamflows. 



Flooding 



Flooding affects all parts of the United States — 

 in arid as well as humid areas. In 1975, despite modern 

 communications and weather services, 113 people 

 were killed by floodwaters, and property damages 

 were estimated at $3.4 billion. Almost half of all flood 

 damages are to agriculture, as crops and livestock 

 are destroyed and production land is covered or 

 washed away. In urban areas, property damage is 

 accompanied by unemployment and dislocation of 

 people. 



The impact of flooding on wildlife, fish, and eco- 

 system is mixed. In upstream areas, wildlife food and 

 habitat are often washed away or covered by flood- 

 waters, resulting in severe damage to natural systems. 

 Less measurable losses include funds spent for relief 

 and reconstruction, lost productivity, and the general 

 disruption of the economy during and after a flood. 

 However, flooding may transport beneficial nutrients 

 that improve or supply natural downstream systems. 



Since 1941, annual flood damages have not been 

 less than $50 million. Yearly damages usually range 

 from $100 million to $400 million. Damages ap- 

 proaching $1 billion have occurred several times 

 since 1950, the highest being $4.5 billion in 1972. 

 Despite the increasing trend in annual flood dam- 

 ages, there is no evidence that storms are increasing 

 in magnitude or frequency. The increases in damage 

 result from inflation and, more importantly, from 

 new development in flood-prone or flood-susceptible 

 areas. ^ 



'■U.S. Water Resources Council. The 1975 assessment of water 

 and related land resources, op. cii. 



Average annual flood damage per square mile 

 varies considerably by region and subregion (fig. 

 7.10). This wide variation in average flood damages 

 is related in part to weather patterns, in part to the 

 character of the streams in the region or subregion, 

 and in part to the average value of property subjected 

 to flooding. 



Floods cause serious health problems, injuries, 

 exposures, stress, and bacterial contamination. Many 

 of these problems may continue long after the flood 

 has subsided. The yearly loss of life from floods has 

 usually been less than 100, but it exceeded 500 in 

 1972. 



Floods can be both devastating or beneficial to 

 agricultural interests. They can wipe out crops and 

 dump tons of sand, gravel, clay, and other debris 

 on productive lands. Floatable debris in flood plains 

 can cause significant damage to structures such as 

 bridges, culverts (and associated roads), and other 

 structures within the flood plain, particularly for 

 floods with recurrence intervals of up to 25 years. 

 Loose materials that are picked up and carried by 

 floodwaters are often trapped against structures such 

 as bridges where they collect and form debris dams. 

 These dams force water to find an alternate route 

 around them. If the debris dams break loose and 

 wash out, the resulting surge of water and debris can 

 cause additional damage to other downstream struc- 

 tures and possible loss of life. On the positive side, 

 slow-moving floods can deposit fertile, highly produc- 

 tive soil on cropland. Other types of enrichment can 

 be found in wetlands and other natural areas where 

 periodic flooding can rejuvenate feeding and breeding 

 areas. 



It is projected that average annual flood damages 

 will increase to $4.3 biUion in the year 2000. Agri- 

 cultural damages are expected to be more than $1.7 

 billion in 2000 while urban damages are projected to 

 increase by 36 percent to $1.6 billion. All other dam- 

 ages are expected to average about $1 billion. The 

 annual loss of lives has varied widely over the years; 

 consequently, no estimates were projected. 



Generally, the regional estimates and projections 

 of flood damages are closely correlated with popula- 

 tion densities. The highest damages are likely to occur 

 in the South Atlantic-Gulf, California, and Missouri 

 regions. Agricultural damages are most important in 

 the South Atlantic-Gulf and Missouri regions, but 

 are also significant in the Upper and Lower Mississippi, 

 Arkansas-White-Red, and the Texas Gulf regions. 

 Urban damages will be more prominent in California, 

 New England, Mid-Atlantic, and the Great Lakes 

 regions. 



296 



