This conclusion requires an explanation that can be based on the geomorphic history of
Chippewa River. For example, there is a significant morphologic feature on the Chippewa
River floodplain, Buffalo Slough, which occupies the southeastern edge of the floodplain
(Figure 5.23). It is a sinuous remnant of the Chippewa River that was abandoned, and it is
evidence of a major channel change in the Chippewa River valley (Table 5.6).
Flow through Buffalo Slough has decreased during historic time, and indeed flow was
completely eliminated in 1876, when the upstream end of Buffalo Slough was permanently
blocked. The abandonment of the former Buffalo Slough channel by the Chippewa River is
the result of an avulsion, but one that took many years to complete. The channel shifted
from Buffalo Slough to a straighter, steeper course along the northwestern edge of the
floodplain. This more efficient route gradually captured more and more of the total
discharge. The new Chippewa channel produced a braided configuration due to a higher
flow velocity and the resulting bank erosion. As the new channel grew, the old course
deteriorated and eventually its discharge was so reduced that the Mississippi was able to
effectively dam the old channel mouth with natural levees and plug its outlet.
The sinuosity of Buffalo Slough is approximately 1.28. Although sinuosity is usually defined
as the ratio of stream length to valley length, it is also the ratio of valley slope to channel
slope. A sinuosity of 1.28 is, therefore, the ratio of the present valley slope (0.00035) to a
channel slope of about 0.0027. This channel slope value is very similar to the channel slope
of the meandering reach upstream of Durand (0.00028) therefore, the meandering pattern of
the Buffalo Slough channel was appropriate. Although delta construction at the Mississippi
River confluence was responsible for the lower valley slope of the lower Chippewa River
valley, the Buffalo Slough channel had a gradient that was not appreciably different from that
in the upstream reach. Channel sinuosity decreased from 1.49 to 1.28 in a downstream
direction, thereby maintaining a channel gradient of about 0.00027.
This sinuous channel could not have transported the large amounts of sediment that the
present braided channel carries to the Mississippi River, or it too would have followed a
straight braided course. Therefore, the present sediment load carried by the Chippewa
River is greater than that conveyed by the Buffalo Slough channel, but this is due almost
entirely to the formation of the new straight, steep, braided channel, which is 121 m (397 ft)
wider than the old sinuous Buffalo Slough channel. It appears that the lower Chippewa has
not been able to adjust as yet to its new position and steeper gradient, and the resulting bed
and bank erosion has supplied large amounts of sediment to the Mississippi. The normal
configuration of the lower Chippewa is sinuous, and if it could be induced to assume such a
pattern, the high sediment delivery from the Chippewa might be controlled. An appropriate
means of channel stabilization and sediment load reduction in this case is the development
of a sinuous channel.
Since the above suggestions were made (Schumm and Beathard 1976), more detailed
studies of the Chippewa River basin indicate that upstream sediment production must be
controlled, especially where the upper Chippewa River is cutting into the Pleistocene
outwash terraces. If the contribution of sediment from these sources were reduced, the lower
Chippewa could resume its sinuous course.
An indication that the pattern conversion of the Chippewa could be successful if the
upstream sediment sources were controlled is provided by the Rangitata River of New
Zealand. The Rangitata River is the southern most of the major rivers which traverse the
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