across the meander to shorten the flow line of the river, reduce the flood stage and generally
improve poor hydraulic conditions in that location. Several interesting results developed.
In the vicinity of the cutoff, the bankline eroded and degradation was initiated. Within the
bendway, the small tributaries continued to discharge their water and sediment. Because of
the flat gradient in the bend, this channel section could not convey the sediment from these
small stream systems and aggradation was initiated. Within a short period of time, sufficient
aggradation had occurred so as to jeopardize water intakes and sewage outfalls. As a
consequence of the adverse action in the vicinity of the cutoff and within the bendway itself, it
was finally decided that it would be more beneficial to restore the river to its natural form
through the bend. This action was taken and the serious problems were alleviated.
In such a haphazard program of river development, the highway engineer would be hard
pressed to maintain and plan for the highway system along and over this reach of river.
Another common case occurs with the development of reservoirs for storage and flood control.
These reservoirs serve as traps for the sediment normally flowing through the river system.
With sediment trapped in the reservoir, essentially clear water is released downstream of the
dam site. This clear water has the capacity to transport more sediment than is immediately
available. Consequently the channel begins to supply this deficit with resulting degradation of
the bed or banks. This degradation may significantly affect the safety of bridges in the
immediate vicinity. Again, the degraded or widened main channel causes steeper gradients on
tributary streams in the vicinity of the main channel. The result is degradation in the tributary
streams. It is entirely possible, however that the additional sediments supplied by the tributary
streams would ultimately offset the degradation in the main channel. Thus, it must be
recognized that downstream of storage structures the channel may either aggrade or degrade
and the tributaries will be affected in either case.
Significant responses can be induced upstream of reservoirs as well as downstream. When
the stream flowing into a reservoir encounters the ponded water, its sediment load is deposited
forming a delta. This deposition in the reservoir flattens the gradient of the channel upstream.
The flattening of the upstream channel induces aggradation causing the bed of the river to rise,
threatening highway installations and other facilities. For example, Elephant Butte Reservoir,
built on the Rio Grande in New Mexico, has caused the Rio Grande to aggrade many miles
upstream of the reservoir site. This change in bed level can have very significant effects upon
bridges, other hydraulic structures and all types of training and stabilization works. Ultimately
the river may be subjected to a flow of magnitude sufficient to overflow existing banks, causing
the water to seek an entirely new channel. With the abandonment of the existing channel
there would be a variety of bridges and hydraulic structures that would also be abandoned at
great expense to the public.
The clear-water diversion into South Boulder Creek in Colorado is another example of river
development that affects bridge crossings and encroachments as well as the environment in
general. Originally the North Fork of South Boulder Creek was a small but beautiful scenic
mountain stream. The banks were nicely vegetated; there was a beautiful sequence of riffles
and pools, which had all the attributes of a good fishing habitat. Years ago, water was diverted
from the Western Slope of the Rockies through a tunnel to the North Fork of South Boulder
Creek. The normal stage in that channel was increased by a factor of 4 to 5. The extra water
caused significant bank erosion and channel degradation. In fact, the additional flow gutted
the river valley, changing the channel to a straight raging torrent capable of carrying large
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