8170 - Laserfiche WebLink

Home Browse Search

promising developments have come from the modeling of fluid dynamics and sediment transport [e.g., Seminaza and Tubino, 1989; Nelson and Smith, 1989a,b]. Leopold et al. [ 1964] noted that the form of meanders varies greatly, and that the amplitude of meanders was probably related to the erosional characteristics of the bank sediments. Ikeda [ 1989] investigated the sedimentary controls on river form and classified meandering rivers into four types: (1) fixed meanders, (2) restricted meanders, (3) confined free meanders, and (4) true free meanders. The meanders of the Green River through the central Uinta Basin and Ouray NWR are restricted by resistant deposits of Pleistocene age [Schmidt, 1994]. Ikeda [1989] determined that bar form and characteristic in restricted meander reaches aze products of the meandering flow rather than causing the meandering flow. The Green River in the Ouray NWR is, at high flow, a meandering, single-threaded channel between two well defined banks. In some years, the Green River at low flow is multithreaded, with flow divided by emergent midchannel bars. Consequently, the Green River exhibits characteristics of both meandering and braided rivers. Bars There are two dominant research approaches that have considered barforms. One approach has focused on describing the topographic and sedimentologic features of different systems. Another approach is to determine the physical processes that produce baz features. Geomorphic reseazch continues on river bedforms [e.g., Rajaguru et al., 1995; Bridge and Gabel, 1992; Gabel, 1993; Brierley, 1991; Rubin et al., 1990; Crowley, 1983], and the debate on the classification and naming of those features also continues. Many reseazchers consider the bedforms between ripples and upper plane bed as a continuum of what should be called "dunes" [Ashley, 1990]. These "dunes" have wide-ranging spacing from 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 bed- and barforms of importance to this study. In sand-bedded rivers, many large-scale geomorphic sediment features form at higher "channel-forming" flows and are exposed by subsequent lower flows [Crowley, 1983]. Many forms of these within-channel features, such as bars, exist [Brierley, 1991 ]. Bars have lengths of the same order as the channel width, and heights of the same order as the 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 et al., 1964]. Alternate bazs 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 et al., 1964; Ikeda, 1989]. Midchannel bars, typically found in braided rivers but also in some meandering rivers, are roughly diamond shaped, and align with and split the low flow [Leopold et al., 1964]. Midchannel bars are transient in nature, but point bars are stationary features. The midchannel and some superimposed bars of the Green River within Ouray NWR aze similar to those described by Cant and Walker [1978] in the braided South Saskatchewan River of Canada. Portions of cross- channel bars emergent at low flow accrete additional sediment at low flow, creating sand flats just above base flow level. In this research, I use the terminology of Brierley [1991] to describe some distinguishing chazacteristics of these bazs (Figure 2). Chute channels are any short-circuiting channel across a A-4 ~' 1 1 r~~ _~ ~-' ~' 1