ature usually about midway between. The water temperature usually approaches the minimum 

 air temperature In early spring and In late fall, and the maximum air temperature dirrlng 

 summer. The water reached a maximum temperature of 85 degrees F. In the first week of 

 July. Later, In the middle of September, when the air temperafare reached Its maximum 

 for the year, the lake water was cooler than In July because of lov/er air temperatures 

 during the night. The bulb of the recording thermograph was set at a depth of about 3 

 feet because we estimate that this Is the average depth at which fish live in this lake 

 during the growing season. 



Although many things are known to affect the growth of fishes, low water temperature 

 and a short growing season are among the most Important limiting factors on the attain- 

 ment of the highest possible fish production. One important reason is that fish are cold- 

 blooded animals and cannot digest food or grow rapidly unless the water is above 55 or 60 

 degrees F. 



Markus (1932) found that the rate of digestion in bass is very slow in water below 55 

 degrees F., but that it increases rapidly with Increasing temperatures up to 80 or 85 

 degrees F. The water temperature of Fork Lake rose above 55 degrees F. during the last 

 week in April and stayed above until the last week in October, These dates define an ac- 

 tive growing season of 6 months in 1939. It may be noted that the average total lengths 

 of fish taken in November were no greater than of those taken in October, fig. 1. The 

 apparent growth of the 1938 brood of bluegills previous to the last week in April was due 

 probably to the elimination of the smaller individuals by bass. 



At the bottom of fig. 9 is shov/n the average rainfall in inches, by weelcs. The amount 

 and distribution of rainfall are very Important since they affect both the water level and 

 transparency. Fork Lake has a drainage area of about 60 acres, and the soil in the basin 

 is a tight clay loam. Although erosion is well controlled, a certain amount of clay enters 

 the lake after heavy rains. Early in 1939, the soil was saturated with water, and the rate 

 of run-off was high. On March 1, the lake was full (5 feet on the gage board). On March 

 4, a small leak developed under the spillway. This leak was repaired temporarily on March 

 6, but water broke through again and the leak was not permanently repaired until a week 

 later, when the water level had dropped 0.8 foot below the spillway crest. Although no 

 more heavy rains occurred until the week of April 10, the lake level gradually rose as a 

 result of light rains and seepage from surrounding land. Following the heavy rains of 

 April 10-16, the lake filled and water ran over the spillway. The lake level dropped 

 slowly but steadily throughout late spring and early summer, even though there was as much 

 as 2 Inches of rainfall per week. However, heavy rains in August raised the water 0.3 

 foot. After August, the water level gradually dropped to a minimum of 1.8 feet on the gage 

 at the end of December. 



Fork Lake, with about 43 acres of drainage area per acre of water surface and with 

 average rainfall, may be expected to drop about 3 feet below the crest of the spillway in 

 autumn. 



The transparency varied from 0.45 to 4.8 feet, as measured by a Secchi disc. During 

 the week of March 6, 3.86 Inches of rain fell, and the transparency the following week was ■ 

 the lowest of the year. Throughout the year transparency seemed to be inversely propor- 

 tional to the amount of run-off, with a lag of a few days which increased late in the 

 summer when the groimd was dry. 



A dry spell the latter part of July and early August was accompanied by a "bloom" of 

 algae which reduced the transparency. This "bloom" was composed of dinoflagellates. On 

 August 14, the lake was stirred with an outboard motor for an hour. After a few days 

 there was a marked Increase in the transparency. Rain later in August again made the lake 

 turbid. A severe wind storm on September 15 stirred the lake completely and was follovred 

 after 3 days by an increase in transparency due to the reduction in plankton called by the 

 disturbance of summer stratification. 



Eleven vertical water temperature sections were made between late March and late 

 October. These temperatiu'es are recorded in table 11. They show a definite stratifica- 



TABLE 11. —VERTICAL WATER TEMPERATURE SECTIONS, DEGREES F.,-IN FORK LAKE DURING 1939. 



15 



