164 
rant hydrographs are found. In the discussion which follows, 
the average or “normal” conditions and figures pertaining 
thereto are based upon the twenty-one years of record at Cop- 
peras Creek, eighteen miles above Havana. 
The year 1894 (Pl. VIII.) is typical in that the high- and 
low-water periods are normally located as to season and also 
in the presence of a March, June, and September rise. Both 
the extreme and average heights for the year, 10.4 and 4.638 
feet respectively, are, however, much below the general aver- 
age (13.8 and 6.74 feet). The high-water period (above 8 feet) 
is shortened to three weeks, and the overflow stage is thus al- 
most eliminated. The concentration of the sewage in the nar- 
row limits of the channel during the early summer favors the 
greater development of the plankton. With the exception of 
the September rise the extreme low water continued without 
interruption for a period of eight months—till the last of Feb- 
ruary, 1895. These are conditions which cause the drying up 
of extensive backwater areas, and also the development of a 
large amount of aquatic vegetation in those lakes and marshes 
which remain—a circumstance which reduces their plankton, 
and their contribution, if there be any, tothe river. The auton- 
omy of the river plankton is thus emphasized in such a year 
as this, which may be briefly characterized as one of predom- 
inant low water and unusually stable conditions. 
In 1895 (Plate IX.), another low-water year, we find, on 
the other hand, little that approaches the normal. There is, to 
be sure, a diminutive March rise and a sharp but very brief 
equinoctial one, with very low water in the autumnal period. 
The abnormal features are the failure of overflow, the long low 
stages,—almost ten months, with unusually low water in Feb- 
ruary and June,—the July rise, and the December overflow. 
The extreme low water of the year is apparent in the average, 
3.61 feet, the lowest on record in twenty-one years. The low 
water in the winter combined with ice produced a stagnation 
fatal to the plankton, while the June minimum favored an un- 
usually large development for that season. The July, Septem- 
ber, and December rises flushed out the river. The low water 
