FLOATING AND RAFTING 
399 
Pier Dams and Abutments 
Pier dams arc cribwork structures used to narrow the channel 
of a stream, guide logs past rocks and other obstructions, and 
in some cases to block an old 
channel and divert the water into 
another course. 
They resemble the piers of crib 
dams having cribs from 6 to 8 
feet square, and mud-sills fastened 
to bedrock or firmly anchored in 
the stream bed. The cribs are 
loaded with rock to give them 
stability. 
Abutments are used to pro- 
tect the banks of streams during 
flood time, and prevent them 
from being worn awa}'. The usual form is a cribwork of 
timber built into the bank. The space between the shore and 
the timbers is filled with rock to prevent the bank earth from 
washing out. Where streams pass through wide bottoms and 
the banks are too low to confine the flood water, an artificial 
Fig. 138. — An Abutment for the 
Protection of Stream Banks. 
Fig. 139. — An Artificial Channel used to confine Flood Water in a 
Narrow Bed. 
channel is sometimes created by constructing false banks of 
lumber. Cribwork supports a strong frame of timbers on which 
heavy planking is nailed. 
Boom^. — Backwaters, pockets, low banks, obstructions and 
shallow places where logs are apt to be lost or stranded occur 
on most streams. Booms, made of long sticks of timber 
fastened together end to end and moored to objects on shore or 
to piling or cribs in the stream, are used to confine the logs to the 
channel. Booms are also used to aid drivers in sluicing logs 
through dams, for confining logs at assorting gaps and storage 
