34 
Transactions Texas Academy oe Science. 
[80] 
"This dam was built on the rock bed of an ancient channel of a great 
river. Both ancient shores are of rock and nearly vertical to the height 
of the dam. When the dam was constructed the modern river occupied 
less than half the ancient river channel, and the remainder of the chan- 
nel, covering somewhat more than its easterly half, was occupied by an 
alluvial deposit forty to sixty feet in depth. A narrow cut was made 
through this deposit to the east shore for placing the foundations of the 
dam in that part of the ancient channel. The tail water from the power 
house flowed out through this cut on bed rock to the modern west channel, 
as shown in Fig. 11, and had the toe of the dam for its right shore and 
the earth deposit for the other: shore. 
“In examining the theory of the bed rock cutting by the tail water 
alone we observe that the quantity of tail water flow was ordinarily about 
250 cubic feet per second, and in the narrowest part of the channel had 
a velocity of about two feet per second. In the wide section of this chan- 
nel, at point of scour, the tail water alone had a mean velocity less than 
three-tenths feet per second. The theory of scour and undercutting of 
til'd rock by the tail water flowing at these low velocities is absurdly 
erroneous. 
“The undercutting was probably not done by the scour of extreme 
floods. It was anticipated that the cutting by floods would be at a dis- 
tance from the ice of the dam. Figure 11 shows that floods passed over 
a space in front of the toe of the dam and did their cutting of the alluvial 
tfeposit below the line of the lower end of the power house, about 200 
feet from the toe of the dam. 
“When a flood glides down any sloping face on the lower side of a dam 
its current is discharged in solid stream under the back water below the 
