166 



waters is accompanied by a considerable increase in their 

 plankton. Tliis fact, combined with the increase in the vol- 

 ume of their contributions due to higher levels, augments the 

 relative importance of their share in the formation of the river 

 plankton, tending to increase its quantity and variety. On the 

 other hand, the repetition of floods, no less than eight of which 

 may be found in the hydrograph, flushes the river so often that 

 no concentration of sewage and marked maximum of plankton 

 occur. The unusual extent of these movements is apparent 

 when the total movement for the year, 50.7 feet, is compared 

 with the totals of other years having about the same average 

 height. For example, 1890 and 1897, with an average height 

 of 6.9 feet, have a total movement of only 44.2 and 36.56 feet 

 respectively. In brief, the year was one without extended 

 overflow, with lower water than usual at the normal flood sea- 

 son, with prolonged bank-full river and reservoir action of the 

 permanent backwaters, and with more than the usual turmoil. 

 In 1897 we And a hydrograph (PL XI.) which approaches 

 the mean closely in its main features, and exhibits all the ex- 

 pected movements excepting the equinoctial rise. The average 

 height for the year, 6.90 (6.86 at Copperas Creek), is also near 

 the general average (6.74). The year thus approximates the 

 normal. The high-water period is of 141 days' duration, al- 

 most exactly the average (140), but it occurs somewhat earlier 

 in the year and attains 16 feet — a little more than the usual 

 height. The earlier decline renders more prominent the June 

 rise, and gives an early start to the extreme and uninterrupted 

 low water of the remaining five months of the year. The low- 

 water period (155 days) is normally located but is somewhat in 

 excess of the average (147), and it is also unusual in the fact 

 that the extreme low-water level (1.7 feet since the completion 

 of the dam at LaCrange) continued almost unchanged from 

 the middle of August till the first of November. This was fol- 

 lowed by the usual slight increase in water in the closing 

 months of the year. The total movement of the year (43.1 

 feet) is considerable in view of the average height (6.9 feet), 



