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<br />Borrow pits, depressions, marshes, and sloughs which will be <br />flooded when the reservoir is at maximum pool level and which will <br />retain water at lower pool levels may be serio~s mosquito sources. <br />Drainage ditches should be provided for all such areas, particu- <br />larly those located within the normal summer fluctuation zone. <br />These ditches should insure complete drainage of the areas when <br />the reservoir recedes and permit fluctuation of water levels in <br />them when the reservoir is at higher elevation. Such drainage <br />ditches may also serve to prevent the stranding of fish when the <br />lake recedes. Logs, drift, and other debris should be removed <br />from natural drainageways and existing ditches where such work <br />is indicated for mosquito prevention and control. Permanent shore- <br />line improvement measures can sometimes be employed to eliminate <br />mosquito-producing areas in reservoirs when the normal summer <br />fluctuation zone is limited to a few feet. <br /> <br />Since the effectiveness of naturalistic, preventive, and <br />permanent mosquito control measures is determined by the amount <br />of clean water surface they create, it is essential that ponds <br />and reservoirs be properly maintained after they have been im- <br />pounded. Furthermore, maximum mosquito control benefits from <br />water level management can be realized only when impoundments <br />are properly prepared and maintained. Several phases of water <br />level control, such as flood surcharge, constant level pool, <br />cyclical fluctuation, and/or seasonal recession, can sometimes <br />be adapted to various types of impoundments in order to minimize <br />conditions favorable for mosquito production. Certain phases may <br />be used to control vegetation and flotage or they may be used <br />primarily to destroy mosquito larvae. A flood surcharge above <br />the normal pool level for a few days in the spring prior to the <br />mosquito production season followed by a sharp drawdown is <br />effective in stranding objectionable flotage. The invasion of <br />marginal vegetation can be limited by holding the reservoir at a <br />constant level during the early spring growth period. This phase <br />is especially important since it increases the effectiveness of <br />subsequent fluctuation and/or recession. Seasonal water level <br />recession alone or in combination with periodic fluctuation is <br />an effective control measure since ,it provides a clean shoreline <br />which is unfavorable for mosquito production. In some irrigation <br />storage reservoirs, the normal water releases during the growing <br />season usually result in a drawdown that keeps the pool level <br />below encroaching marginal vegetation, thereby providing a clean <br />shoreline that is unfavorable for mosquito production. <br /> <br />Recreational areas, especially those with facilities where <br />people may remain overnight, should be located along sections of <br />the reservoir which have the lowest potential for the production <br />of mosquitoes and other aquatic insects of public health importance. <br /> <br />C-2 <br />