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 1883, 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 (PI. II. and III.) in the latter 



