8251

• UCREFRP Catalog Number: 8251
• Author: Rakowski, C. L. and J. C. Schmidt.
• Title: The Geomorphic Basis of Colorado Squawfish Nursery Habitat in the Green River Near Ouray, Utah.
• USFW Year: 1996.
• USFW - Doc Type: #93-1070,
• Copyright Material: NO

Draft Final Completion Report to UDWR for Contract #93-1070, Amendment 3

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determining the physical processes that produce bar forms of different morphology.

While geomorphic research continues on river bedforms (for example, Rajagura and others, 1995; Bridge and

Gabel, 1992; Gabel, 1993; Brierley, 1991; Rubin and others, 1990; Crowley, 1983), the debate on the classification and

naming of those features also continues. Many researchers consider the bedforms between ripples and upper plane bed

as a continuum of what should be called "dunes" (Ashley, 1990). These "dunes" have spacing of less than 1 m to

greater than 1000 m. However, the terminology commonly understood by ecologists and geomorphologists is defined in

the following paragraphs and used to describe the geomorphic features of the study area.

In sand-bedded rivers, many large-scale geomorphic sediment features form at higher "channel-forming" flows

and are exposed by the subsequent lower flows (Crowley, 1983). Many forms of these within-channel features (i.e.,

bars) exist (Brierley, 1991). Bars have lengths of the same order as the channel width, and heights of the same order as

mean depth of the generating flow (yalin. 1992). Alternate bars form in both straight and meandering channels, and

occur periodically along alternating banks as the thalweg meanders between bars at low flows (Leopold and others,

1964). Alternate bars may migrate in the downstream direction or they may be fixed in their location. Point bars occur

on the inside of meander bends, and are in part, a product of helical flow in the bend (Leopold and others, 1964; Ikeda,

1989). Mid-channel bars, typically found in braided rivers but also in some meandering rivers, are roughly riillmond

shaped, and align with and split the flow (Leopold ~ others, 1964). While mid-channel bars are transient in nature,

point bars are stationary features.

In this research. the terminology of Brierley (1991) is adapted to describe some distinguishing characteristics

of these bars (Fig. 8). Chu te channels are any short -circuiting channel across a bar. A secondary channel is a chute

channel that occurs on the shoreward side of a bank-attached alternate bar. Bank-attached bars occur along the margins

of the channel although at some discharges these bars may be separated from the bank by secondary channels. Bank-

attached bars with laterally dissecting chute channels are referred to as bank-attached compound bars, even if the chute

channels are inactive at some discharges. Point bar are associated with meander bends.

Bars within a channel migrate downstream or are stationary in location. Within the Ouray NWR, the location

of bars has been relatively constant over the last 30 years (Andrews and Nelson. 1989). Ikeda (1989) described two

types of stationary bars observed for rivers with restricted meanders: fixed and forced. Ikeda used flume study results

to explain the processes that produce each bar type. Migrating alternate bars become fixed when the deflection angle of