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. IT.) 
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 ict. 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 toa 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. 
