32 



Usinger: Introduction 



production of aquatic insects but also influence the 

 growth of marginal vegetation and consequently the 

 cost of maintenance. In planning a water level sched- 

 ule for a reservoir it is necessary to consider: 



a. The primary purpose or purposes of the project — 

 whether for flood control, power, navigation, 

 water supply, irrigation, recreation, wildlife, or 

 a combination of these. 



b. The stream flow and volume of storage in the 

 fluctuation zone — the probability of filling each 

 year, amount of fluctuation possible, and the 

 seasonal recession are all functions of these 

 two items. 



c. The design of the dam and the water level control 

 facilities — the maximum pool level, flood sur- 

 charge, and rate and extent of recession, all 

 depend upon the type of design, elevation, and 

 capacity of the control facilities. 



d. The topography and vegetation in the fluctuation 

 zone — a steep, rugged shore line exposed to 

 wave action will require much less precise water 

 level management than a shore line of extensive, 

 flat, shallow areas; and water level fluctuation 

 is sometimes ineffective for mosquito control 

 where the shore line or margin is colonized with 

 certain types of marginal or aquatic vegetation. 



The simplest schedule for high storage reservoirs 

 with steep banks and little or no mosquito production 

 is direct seasonal recession. For large reservoirs at 

 low elevations a more complicated schedule may be 

 required. The classical example of this is the com- 

 bination of seasonal recession and cyclical fluctuation 

 (intro. fig. 48) used on twenty-four of the large Ten- 

 nessee Valley Authority impoundments involving 

 735,000 acres of water surface and 10,000 miles of 

 shore line. This schedule, which is not directly 

 applicable to California conditions, calls for maximum 

 elevation for a short time before April 1 to strand 

 the winter accumulation of drift and floatage. Then 

 there follows a constant pool level during the spring 

 growth period (April 1 to May 15). This prevents the 

 invasion of marginal vegetation into the zone of 

 fluctuation and delays the germination of annual 



Winter Period 



Controlled Elevation' 

 not Necessary for 

 Mosquito-Control 



Spring 

 Growth 

 Period 



Maximum 

 Mosquito- 

 Control 

 Elevation 



Malaria Mosquito Production Period 



Maximum Elevation— flood Surcharge 



w 



Maximum Mosquito-Control Elevation 



i for Navigation and Power 



Minimum in Advance of Floods 



Cyclical 

 Fluctuation 



Larviqdes 



Seasonal Recession 

 Cyclical Fluctuation 

 larvicides 



Fall Low Rainfall Period 



Recession About 0. 1 Foot per Week 



Basic Clearing Line 



Cyclical Fluctuation Approximately : _ 

 1-Foot at Weekly or 10-Day Intervals 

 I 



Apr 1 Mi, 



Approximate 



M» IS lal I Sep I Oct I 



Dates Vary with Location and from Season to Season 



Intro, fig. 48. Schedule for water level management on main 

 river reservoirs of the Tennessee Valley Authority, combining 

 cyclical fluctuation and seasonal recession (after Hess and 

 Kiker, 1944). 



plants, thus decreasing the cost of annual shore line 

 conditioning. During the period of moderate mosquito 

 production (May 15 to July 1) the water level is raised 

 and lowered one foot at weekly or ten-day intervals. 

 This alternately strands larvae and eggs on the shore ; 

 or flushes them out of protective vegetation and ' 

 exposes them to predators. During the period of 

 heaviest mosquito breeding (July 1 to October 1) 

 cyclical fluctuation is combined with an over-all 

 recession of about 0.1 foot per week. Finally (after 

 October 1) during the period of low rainfall the water 

 level is lowered to or near the minimum required for 

 navigation and power. This exposes broad expanses 

 of shore line for the annual job of shore-line con- 

 ditioning. 



4. Preimpoundage studies should also be made of 

 the productivity of lakes and reservoirs in the vicinity 

 as an indication of the probable adequacy of fish-food 

 organisms. In this connection a fish-stocking plan 

 may be developed to ensure the best utilization of 

 fish-food organisms in the reservoir. 



5. The productivity of the stream below the proposed 

 dam should be determined as accurately as possible 

 in order to predict the effects of periodic flooding 

 or drying and to recommend the optimum flow for 

 maintenance of adequate bottom food organisms for 

 fish. Such figures are often used as a basis for legal 

 action when adverse effects are noted after impoundage. 



Duck ponds. — The impoundment of water by clubs 

 for duck hunting has become a common practice in 

 parts of California. The subject was difficult in the 

 past because the interests of sportsmen and mosquito 

 control agencies appeared to be in conflict. More 

 recently it has become evident that good practices 

 for duck clubs are also good for mosquito control. 

 After considerable study the Wildlife Committee of 

 the California Mosquito Control Association made 

 the following recommendations pertaining to the 

 management of duck ponds: 



1. If it is desired to hold ducks in a hunting area, 

 a certain proportion of the area should be prepared 

 as good sharp banked and properly maintained ponds, 

 and permanently flooded rather than flooding the whole 

 area early in the summer to achieve this purpose 



2. An attempt should be made to adjust the duck 

 hunting season by smaller regions allowing for a later 

 season in the Central Valley. This is concurred in by 

 many hunters; however, the setting of the season is 

 done by the United States Fish and Wildlife Service 

 and direct recommendations will have to be made by 

 them. 



3. Efforts should be made to control or eliminate 

 cattails, tules, and other emergent vegetation, espe- 

 cially in permanently flooded ponds. 



4. Where seepage areas occur outside of ponds a 

 ditch should be dug around the pond to cut off this 

 water and the water in the drain ditch should be 

 disposed of by draining into an existing drainage 

 system or pumping back into the pond. 



5. Ponds should be drained immediately after the 

 close of the season. 



6. The planting of food grains in the ponds should 

 be done in such a manner that no mosquito problems 

 can be caused. 



