A heavy rain can cause problems but also is an important part of the renewable water supply picture. Since rivers in the area are very different than they were 150 years ago, floods and natural recharge occur differently now than in the past. Increased urbanization, use of flood control structures, natural recharge and artificial recharge are interrelated issues that need careful consideration. Water quality problems can arise when urban runoff carries various pollutants from paved areas to riverbeds; some of the runoff could then enter the groundwater |
FLOODING, THEN AND NOW
What is a flood? According to Websterâ€™s dictionary, a flood occurs when water overflows onto normally dry land. In this arid region the term often is used to describe a situation when an unusual amount of water flows in usually dry rivers, whether or not it overflows the banks. In Arizona an unusual amount of water flowing in a river with sufficient force to erode its banks is almost invariably referred to as a flood.
Floods provide both benefits and problems. Floods can be beneficial when they recharge groundwater, but the same flow also can damage buildings and roads, erode land, and carry pollutants that may reach the groundwater.
Years of bountiful rain and snow alternate with years of little precipitation. Heavy river flows occur occasionally and are essential for the growth of riparian vegetation. During flood years new cottonwood seedlings sprout and take hold. Traditional farming took advantage of the summer rainy season when water overflowed river banks bringing both moisture and nutrients to the soils. River flows replenished the groundwater table that remained near the surface. The water table was sufficiently high along most of the Santa Cruz River, from San Xavier to the CaÃ±ada del Oro, that cottonwood forests thrived. Such forests also were located along parts of the Rillito Creek, Tanque Verde Wash and Pantano Wash. A giant mesquite bosque flourished south of San Xavier.
Changed river conditions have destroyed much of this riparian habitat, and developing new riparian habitat would be a difficult challenge, even using CAP water and effluent. Some even argue that conditions have deteriorated to the point that many riparian areas are beyond restoration. Construction along riverbanks and flood control structures have radically changed natural river conditions. Instead of meandering and spreading out onto floodplains to benefit riparian vegetation, floods now often are contained within deep channels.
The occurrence and intensity of flooding in southern Arizona appears to have increased during the last 30 years. While periodic changes in weather affect floods to a degree, human factors have played a greater role in determining flood damage. Floods that occur in wilderness areas are hardly noticed because humans usually are not affected. Floods that occur in urban areas, however, can affect large numbers of people and thus attract attention.
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