LIMNOLOGICAL OBSERVATIONS IN THE UPPER MISSIS- SIPPI, 1921. By P.- S. GALTSOFF, Naturalist, U. S. Bureau of Fisheries 'Steamer Albatross. CONTENTS. Introduction.............................. Method of investigation .-................. Schedule of localities ................. Boat and equipment. ................. Instruments . ................... Pump..... ................ Determination of plankton............. Stations ............................ Physiography ............................ The river............................. Lake Pepin.......................... Lake St. Croix...-.................... Lake Keokuk......................... Stages.......................... Velocity of current............ ....... Discharge . ........................ Flow of sediment . .................. Chemical constituents of water......... Transparency of water............... Temperature of water ................ Page. 347 351 351 353 353 354 354 355 357 357 359 360 360 362 366 369 370 371 371 373 Plankton and detritus .................. Volume.............- - - -.............. Horizontal distribution in river ........ Vertical distribution in river -......... Lake Keokuk ........................ Lake Pepin........................... Lake St. Croix....-......... ......... Pump and net collections.............. Distribution of Copepoda and Cladocera.... The river ................... ......... Lake Keokuk .,......... ............ Lake Pepin-.......................... Composition of the plankton............... The river. -...........--....-........ Lakes.....-............. ............. Tributaries.....-.....-.....-......... Discussion and conclusion . .............. Potamoplankton ...................... Plankton and the fisheries............ Summary............................. Bibliography ............ ............... INTRODUCTION. The present paper deals with the results of the hydrobiological investigation of the section of the upper Mississippi between Hastings, Minn., and Alexandria, Mo., which is about 465 miles long if measured along the steamboat channel. At the extreme ends of this section the river forms two lakes-Lake Pepin and Lake Keokuk (fig. 3, p. 352). Lake Pepin, located 28 miles below Hastings, is a natural lake about 25 miles long and from 1 to 3 miles wide. Lake Keokuk is a recently formed basin that extends northward above the Keokuk Dam for about 60 miles. Between these two lakes, about 130 miles above the foot of Keokuk Lake and 240 miles below the foot of Lake Pepin, the river flows through the so-called Rock Island Rapids. The bed of the river here forms a series of steps causing rapids with a total fall of 21 feet in 16 miles. The character of the river above and below the 347 Page. 375 375 379 379 380 382 384 385 387 387 389 394 396 403 406 410 411 411 415 420 434