WSP03817

• File Number: 8102
• Description: Arkansas River Basin Basic Hydrology
• State: CO
• Basin: Arkansas
• Water Division: 2
• Date: 5/7/1999
• Author: Inter Fluve Inc
• Title: Fluvial Geomorphic Assessment of Upper Arkansas River Final Report
• Water Supply Pro - Doc Type: Report/Study

I
I
I
I
I
I
I
I
I
I
,
I
I

I
I
I
I
I

['18 9
6. Geomorphic Evolution of Project Reach

The Upper Arkansas River has experienced historic alterations in both hydrology and
sediment load since 1850. Mining in the watershed began in the 1850's and resulted in
the delivery of relatively fine-grained tailings to the system. Local placering may have
also introduced relatively coarse material. Approximately 100 years later, the onset of
flow augmentations resulted in significant erosion in upstream tributaries, which may
have increased sediment loads as well as flows to the project reach (Chapter 3; Dominick
and O'Neill, 1998). The increased sediment loads and flows likely resulted in
accelerated rates of channel migration and avulsion. The complex series of impacts over
150 years make it exceedingly difficult to detennine the specific channel response to each
impact based on the limited data available. Consequently. the attempt to decipher the
geomorphic response of the system to imposed change consists ofa detennination of
general channel evolutionary trends that should be considered in any long-tenn
rehabilitation plan.

6.1. The Stable Channel Balance

The Upper Arkansas River is an alluvial river, which means that its bed and banks are
composed of sediment that were deposited by the existing stream system. Alluvial rivers
have erodible boundaries, and are free to adjust dimensions, shape, pattern, and gradient
in response to change in sediment supply and hydrologic regime. Since they are
adjustable, alluvial rivers tend toward a stable channel balance, in which the channel is
adjusted to convey the flow and sediment delivered without undergoing significant net
erosion or deposition. This condition is referred to as sediment transport equilibrium, or
geomorphic stability. If general trends in hydrology or sediment load change
significantly within a river system, a geomorphic response to that change should be
expected.

Channel instability commonly results from an imbalance of sediment load and flow
energy, or from unstable topographic configurations such as channel perching above the
valley floor. Historic developments within the Arkansas River system, including mining
and flow diversions, have altered both the background sediment load and hydrology
within the project reach, Both field and experimental investigations have demonstrated
that alluvial rivers respond to changes in discharge and sediment load (Schumm, 1969).
Land use practices such as grazing and floodplain irrigation can result in additional
geomorphic adjustment. The cumulative response to such a series of impacts reflects a
complex sequence of channel adjustments that vary as a function of the timing,
magnitude, and spatial distribution of impacts, as well as the occurrence of hydrologic
events that drive adjustment (Andrews and Nankervis, 1995; Wolman and Miller, 1960).

Schumm (1969, 1977) identified geomorphic impacts on channels with changes in water
and sediment leading to river metamorphosis. In general, an increase in discharge and
sediment load will result in aggradation (deposition), channel instability, and the
fonnation of a wider and shallower channel. The effect of such changes on slope,

May 7. 1999

Fluvial Geomorphological Assessment
Upper Arkansas River

Page 35