151 
relation between the average heights of the water for the year 
and the total movement of levels, though this correlation is by 
no means continuous or uniform, as will be seen, for example, 
on comparison of the average heights and movements for 1895 
and 1896 (3.61 and 7.26 feet; and 51.89 and 53.16 feet). The ab- 
normally low water in 1895 was followed by an unusually early 
rise in December. Had this rise occurred ten days later the 
correlation of average heights and total movements would have 
been more apparent in these years. 
The range in movement of river levels between the highest 
and lowest water of the year as shown in the tables varies at 
Copperas Creek between 9.00 feet in 1889.and 17.70 in 18838, and 
at LaGrange between 8.75 in 1894 and 20.52 in 1883, while the 
extreme range for the period of record at each dam is 19.27 and 
22.92 feet respectively. Somewhat greater fluctuations thus 
occur at the lower dam. 
The preceding discussion of the fluctuations in river levels 
affords evidence for the extreme instability of the environment 
of the plankton with which we are to deal. Indeed the only 
really constant feature seems to be this very instability. The 
mere presentation of the statistics of the fluctuations can give 
but little life or color to the great modifications which these 
changes produce in the physical conditions environing all the 
aquatic life of the river. The great increase in area and vol- 
ume (not far from one hundredfold) which occurs at high 
water (Plate III.) affords an opportunity for a great increase in 
the total production of the plankton, especially when the flood 
period extends into the spring and early summer months, when 
the maximum development of the plankton sometimes occurs. 
The submerged flood-plain also affords the greatest variety of 
conditions of depth, current, vegetation, bottom, light and 
shade, temperature, and sewage, thus favoring the diversifica- 
tion of the plankton locally produced but carried away to some 
extent into the channel by the receding floods. 
A trip by boat across the submerged bottom-lands from the 
Quiver shore to the western bluff (Pl. II. and III.) in the latter 
