176 



winter season, which prevents the culmination of stagnation 

 conditions. Again many of the backwaters are rich in vegeta- 

 tion, and some of them are spring fed at the margins which 

 thus remain open even in the coldest weather. The river itself 

 rarely closes over entirely, air-holes remaining where the cur- 

 rent is rapid. Thus, below the mouth of Spoon River (PI. II.) 

 a large area was usually free from ice even when the river was 

 closed above this point. The currents due to tributary waters, 

 as in Quiver Lake, and to changes in level, as in all impound- 

 ing waters, also tend to prevent stagnation conditions. In 

 spite, however, of these favoring circumstances one catastrophe 

 of this nature did occur in the years of our work at Havana. 

 In the winter of 1894-95 prolonged low water and heavy ice 

 upon the river and lakes combined to render the conditions 

 unfavorable to life in the river, and to some extent in Quiver 

 Lake. Conditions in other localities at this time were not 

 observed. The practical extinction of the plankton and the 

 death of large numbers of fish attended this period of stagnation. 

 The duration of the ice at the various stations in the sev- 

 eral years is indicated at the bottom of the diagrams which 

 give the hydrographs and plankton data of the several stations 

 by black lines of a thickness proportional to the ice. The 

 occurrence of ice in the different years at Havana has varied 

 considerably. No records were made in 1894-95, but from other 

 sources, river stages and weather reports, it seems probable 

 that the river closed in the last days of December, and that 

 the ice continued until the rise of February 25, a period of 

 almost 60 days. In the winter of 1895-96 there was but little 

 ice, the river and backwaters being partially closed only for 

 the first fortnight in January. In 1896-97 the river did not 

 close until after the rise in the early part of January, the ice 

 remaining about one month, going out with the rise of Febru- 

 ary. The lakes, on the other hand, were closed to a large ex- 

 tent throughout December, and again, to varying extents, dur- 

 ing January and a part of February, the current due to high 

 water keeping portions free from ice at times. 



