Received in Exchange from QH I UNIVERSITY OF IOWA STUDIES IN NATURAL HISTORY VOLUME IX 1920-1921 UNIVERSITY OF IOWA 1921 '/p.,. CONTENTS VOLUME IX No. 1. Birge, E. A. and Juday, Chancey. A limnological reconnaissance of West Okoboji. No. 2. Stoner, Dayton. Nesting habits of the hermit thrush in northern Michigan. No. 3. Trowbridge, A. C. The erosional history of the drift- less area. No. 4. Lindsey, A. W. The Hesperioidea of America north of Mexico. No. 5. Clark, A. H., Kathbun, Mary J., Boone, Pearl L., Shoemaker, C. K., Clark, H. L. Reports on the Crinoids, Ophiurans, Brachyura, Tanidacea and Iso- poda, Amphipods, and Echinoidea of the Barbados- Antigua expedition of 1918. FIRST SERIES No. 35 SEPTEMBER, 1920 UNIVERSITY OF IOWA STUDIES STUDIES IN NATURAL HISTORY Volume IX Number 1 A LIMN0L0GICAL RECONNAISSANCE OF WEST 0K0B0JI by E. A. BIRGE and CHANCEY JUDAY PUBLISHED BY THE UNIVERSITY, IOWA CITY Issued monthly throughout the year. Entered at the post office at Iowa City, Iowa, at second class matter. Acceptance for mailing at special rates of postage provided for in section 1103, Act of October 3, 1917, authorized on July 3, 1018. UNIVERSITY OF IOWA STUDIES IN NATURAL HISTORY Professor Charles Cleveland Nutting, M. A., Editor Continuation of Bulletin from the Laboratories of Natural History of the State University of Iowa Volume IX Number 1 A LIMNOLOGICAL RECONNAISSANCE OF WEST 0K0B0JI by E. A. BIRGE and CHANCEY JUDAY PUBLISHED BY THE UNIVERSITY, IOWA CITY A LIMNOLOGICAL RECONNAISSANCE OF WEST OKOBOJI E. A. Birge and Chancey Juday I—SITUATION, AREA, VOLUME Lake Okoboji lies in Dickinson county close to the north- ern boundary of Iowa ; it is situated in T. 99 N ; R. 36 and 37 W. Its approximate position is lat. 43.35° N ; long. 95.13° W. The region adjacent to the lake is thickly studded with other lakes and lakelets. Two lakes of considerable size- East Okoboji and Spirit lakes—are connected with it. Be- sides these there are ten or more bodies of water, which are large enough to bear names, and smaller ponds and pondlets by the score. None of these bodies of water, except Okoboji lake, has any considerable depth. East Okoboji lake reaches a maximum depth of about 8 m. (26 ft.), but most of its area is less than 3 m. (10 ft.) deep. Spirit lake has a maximum depth of 7.6 m. (25 ft.), but much of the area is between 6 m. and 7.5 m (20 ft., 25 ft.) deep. Oko- boji lake has a maximum depth of more than 40 m. (132 1 ft.) and therefore differs widely from its two neighbors both in physical and biological characters. The following paper records the results of a limnological reconnaissance of the lake, July 28-August 2, 1919. The reconnaissance was made as part of the cooperative work done by the United States Bureau of Fisheries in connec- tion with the Wisconsin Geological and Natural History Survey. The present report is issued with the permission of the Bureau. The study was carried on at the Iowa Lake- side Laboratory, to which the authors are indebted for much help and many courtesies. i Dr. Tilton ('16 p. 96) reports a depth of 135 feet. But as nothing is said re- garding the calibration of his line, the maximum may still better stand at that found by the regular survey. : 4 IOWA STUDIES IN NATURAL HISTORY The region about lake Okoboji and the lake itself was surveyed by the civil engineering department of Iowa State College through its summer surveying camp. The camp was held there from 1905 to 1912, and the topography of the region and the hydrography of the larger lakes were surveyed. In 1913 a pamphlet was published by the Col- lege (Ford '13) giving the methods employed, together with maps showing the results. Among the maps are a topo- graphical map of the region on a scale of 1 in. to 5100 ft., and a map giving the soundings in Okoboji lake on a scale of 1 in. to 2500 ft. The College also kindly furnished us with a blue print map of the lake, scale 1 in. to 1000 ft., which was used in all measurements. This map was repro- duced in the Iowa report on lakes and lake beds on a scale of about 1 in. to 2030 ft. In this report about one page of description is given to the lake and its area is stated as 3788 acres (Iowa '16, p. 56, map No. 18) * The area thus given is accepted in this paper. The small scale map of the lake gives all of the sound- ings, numbering 732, or about 48 per sq. km. The large scale map gives a selection of these, numbering 261. The soundings were taken from a motor boat which was kept constantly in motion. Its position at the moment of sound- ing was signalled to two observers on shore who kept the boat constantly in sight with transits. By this method the position of the boat could hardly have been determined with minute accuracy, especially in the deeper water, and the maps give no dot or other similar indication of the precise position of the soundings. It is known that the small area which contains the deepest soundings lies somewhat to the west of the place indicated on the map. Under these con- ditions no attempt was made to plat the soundings omitted on the large scale map. In drawing the contours for the hydrography a 20-foot interval was used, since there are not enough soundings along the shores to warrant the use of a 10-foot interval. The areas included in the several contours were measured with the planimeter, with the following result * Note: Appended to this article as Plate I. A LIMNOLOGICAL RECONNAISSANCE 5 TABLE 1—AREA OF OKOBOJI LAKE—ACRES (From map, scale 1 m.=l,000 ft.) Area at surface 3788 acres " 20 feet 2750 40 .1629 60 973 80 326 100 114 120 9 132 Note: The surface area is taken from the report of the state high- way commission, 1916, p. 56. The contour map and the measurements show that the main slope of the bottom of the lake is fairly uniform to the depth of 80 ft., but that below this depth the slopes are decidedly steeper. In fact, the form of the basin is such as to lead the observer to expect a maximum depth of about 90 ft. or 100 ft. Instead of this, however, a small area ex- tends to a depth of 132 ft., or 30 ft. to 40 ft. more than would be expected. From the data thus secured a hypsographic curve of the lake was platted and from this were derived the areas cor- responding to 5 m., 10 m., etc. From these data there were computed the area, volumes, etc., of the lake in metric units, as shown in the following table. The same data computed in cubic yards, etc., directly from the areas of the 20-ft. levels give substantially the same results. TABLE 2—OKOBOJI LAKE—GENERAL FEATURES Length, 8.79 km; 5.46 mi. Greatest breadth, 4.57 km; 2.84 mi. Direction of main axis, north and south. Area, 1535 ha; 3788 a. Greatest depth, 40.2 m; 132 ft. Mean depth, 12.3 m; 40.4 ft. Volume, 188,340,000 cu. m; 246,340,000 cu. yds. Length of shore line, 29.3 km; 18.2 mi. Shore development, 2.13. Volume development, 0.92. Mean slope of bottom, 1°34'; 2.74%. NOTES: The mean depth is found by dividing the volume by the area of the surface. The volume of the several strata are computed in wWch is the area f the from the formula (a+B+V^Ab)^ A ° upper surface of the stratum, B is the area of the lower surface and H the vertical distance between the surfaces—in this case 5 meters. ; IOWA STUDIES IN NATURAL HISTORY By shore development is meant the ratio of the periphery of the water to the circumference of a circle of equal area. The mean slope is computed from the formula 1 /2 I +I +I +I +In-1 +%I H S=J 1 2 3 n 1*L A~ j In this formula S=mean slope; I , 1^ etc.=length of contour lines from surface to bottom; n=number of such contours; H=maximum depth; A=area of the lake. (See Juday '14, p. 123). TABLE 3—DETAILS OF OKOBOJI LAKE Area Area Volume between Depth Ha. Per cent Stratum contours Cubic Per cent meters of total meters Ha. meters of total 1535 100.0 0-5 330 68,310,000 36.3 5 1205 78.5 5-10 402 49,900,000 26.5 10 803 52.3 10-15 274 33,100,000 17.6 15 529 34.5 15-20 214 20,880,000 11.1 20 315 20.5 20-25 198 10,360,000 5.5 25 117 7.60 25-30 67 4,O20;OOO 2.1 30 49.9 3.25 30-35 35.3 1,520,000 0.8 35 14.6 0.95 35-40.2 14.6 250,000 0.1 40 188,370,000 The volume development is the ratio of the volume of a lake to that of a cone of equal base and height. If the lake were a tank with vertical sides the volume development would be three. Since in this case the volume development is less than 1.00, the volume of the lake is smaller than that of a cone of equal base and altitude.