30 Artificial Aquatic Habitats 



he can read a contour map and estimate land areas. In looking for suitable 

 sites, the planner will find the quadrangle maps of the United States 

 Geological Survey very useful. These maps, which are usually available 

 through the federal or state geological surveys, show land elevations 

 through the use of contour lines. This makes it possible to locate on them 

 sites for lake basins and dams, or if a potential site has been found 

 through field observations, to determine several suitable locations for a 

 dam, and to estimate the amount of land that will drain into the pond 

 or lake once the location for the dam has been decided upon. 



Watershed, Runoff, and Water Manipulation 



Rainfall, slope of land, and vegetative cover vary within certain limits, 

 but a definite relationship exists between the volume capacity of a pond 

 basin and the area of the watershed needed to keep that basin filled. 

 Although usually it is impossible for the layman to calculate the volume 

 capacity of a selected pond basin, he may, by the use of a quadrangle 

 map, arrive at the approximate surface area of the basin and tlien consider 

 this in relation to the area of the watershed. Where soils are relatively 

 tight ( for example, in Illinois, Iowa, and Missouri ) , the approximate limits 

 of range for the watershed are a minimum of 10 acres and a maximum 

 of 50 acres to 1 acre of pond surface assuming a basin of average depth 

 and contour. However, if this index is used and the drainage area is less 

 than 10 acres, insufficient runoff water will be available during dry 

 periods. On the other hand, if the watershed is 50 or more acres, so much 

 water in excess of pond capacity must be passed through the basin that 

 a large and expensive spillway is necessary. Probably the optimum ratio 

 between watershed and pond surface area is in the neighborhood of 20 

 or 25 to 1. Optimum relationships between the drainage area and the 

 pond size and volume vary greatly among parts of the country with 

 differences in rainfall, slope, soil types, vegetative cover, and evaporation 

 rates. 



Small ponds in the north central states having ratios of watershed 

 to pond surface of less than 15 to 1 may be safely constructed with grass 

 waterways to carry off excess water. However, where the watersheds and 

 ponds are larger, spillways are usually constructed of concrete or stone. 



It is unnecessary to screen spillways to prevent the departure of fish 

 from a pond or lake, as only a small fraction of the fish population will 

 leave. Screens across spillways have a way of becoming clogged during 

 floods and sometimes are responsible for washouts of dams. It is more im- 

 portant to prevent fish from moving up over a dam from below, and spill- 

 ways should be planned to provide insurmountable barriers to fish moving 

 upstream. 



There are many problems involved in handling water flowing in and 



