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<br />Acknowledgments
<br />
<br />Hjahnar W. Hjalmarson, USGS, retired,
<br />provided helpful discussions and review of drafts
<br />of the report. John Parker, USGS, provided
<br />photographs, fieldnotes, and guidance in
<br />resurveying the Hassayampa River near Arlington,
<br />and review of drafts of the report. Melissa
<br />Lawrance, Todd Ingersoll, Ann Tillery, Dawn
<br />McDoniel, and Leigh Hubbard, USGS, assisted
<br />with field work. Richard Helton and Sid Alwin,
<br />USGS, and Steve Acquafredda assisted with
<br />graphics. Jeff Phillips, Robert Jarrett, and William
<br />Emmett, retired, of the USGS participated in
<br />helpful discussions. Joseph Tram, FCDMC, and
<br />Russell Cruff, USGS, retired, also provided
<br />reviews of early drafts of this report.
<br />
<br />MECHANISMS OF CHANNEL CHANGE
<br />
<br />Channel change varies over time and space. In
<br />this study, channel change was investigated that
<br />occurred from one streamflow to another and that
<br />occurred over the course of several years or dec-
<br />ades. Most channel change was studied on a reach
<br />scale. A brief description is presented here of the
<br />mechanisms of channel change that were relevant
<br />to this study.
<br />
<br />Channel Scour and Fill
<br />
<br />The beds of alluvial stream channels in nearly
<br />all physiographic and climatic environments are
<br />scoured during large flows (Emmett and Leopold,
<br />1965). Leopold and others (1964) and Emmett and
<br />Leopold (1965) showed that depth of scour on
<br />perennial streams increases as discharge increases
<br />and that scour may occur continuously throughout
<br />a reach of stream. Theoretically, maximum scour
<br />should occur during maximum discharge because
<br />the shear stress and stream power on the stream-
<br />bed usually are greatest during peak discharge.
<br />Consequently, in indirect measurements of
<br />discharge, the common assumption that the
<br />postflood channel represents peak-discharge
<br />channel conditions is questionable.
<br />The relation between changes in bed elevation
<br />and discharge at a particular cross section is
<br />complex. Andrews (1979) examined II cableway
<br />cross sections along the East Fork River in western
<br />Wyoming during a spring peak-runoff season and
<br />determined that sections that scour during high
<br />
<br />flow tended to fill during low flow, and those
<br />sections that fill during high flow tended to scour
<br />during low flow. The cross sections appeared to
<br />change from filling to scouring or scouring to
<br />filling at about bankfull discharge. The fmdings by
<br />Andrews (1979) support the hypothesis of a vel-
<br />ocity or shear-stress reversal where flow compe-
<br />tence in pools exceeds the flow competence over
<br />rimes above a certain discharge (Knighton, 1984).
<br />Colby (1964) discussed many of the mis-
<br />conceptions of scour and fill of sand-bed channels
<br />and showed that scour at a single point in the
<br />stream is not necessarily continuous across the
<br />channel nor over long reaches of the channel.
<br />Also, most observations of channel-bed elevations
<br />are made at gaging stations where flow is typically
<br />restricted laterally, which results in large amounts
<br />of local vertical scour. Colby (1964) also stated
<br />that the total amount of sand transported by a flow
<br />may account for only a few hundredths of a foot of
<br />depth. Foley (1978) likewise showed that most of
<br />the scour and fill that occurs in ephemeral sand-
<br />bed channels in southern Califomia was attributed
<br />to the passing of dune and antidune crests and
<br />troughs. The passing of these bedforms, which
<br />appear as scour and fill at a single cross section, is
<br />not considered scour or fill along the length of the
<br />channel.
<br />Gaging stations in and near Maricopa County,
<br />Arizona, are in channel reaches where scour-and-
<br />fill processes are active. The extent to which these
<br />processes affect stage-discharge relations at these
<br />sites, especially at large discharges, is uncertain.
<br />
<br />Aggradation and Degradation
<br />
<br />Long-term processes also are at work in con-
<br />trol reaches near gaging stations. Channel-bed ele-
<br />vations may increase, or aggrade, over time as
<br />more sediment is delivered to the reach than can
<br />be transported. Conversely, channel beds may
<br />erode or degrade over time as more sediment is
<br />transported from the reach than is being delivered.
<br />The causes of these changes may be natural as in
<br />the event of large-scale wildfIres or climate
<br />change, or they may be anthropogenic causes such
<br />as deforestation, overgrazing, or mining activity.
<br />In the arid and semiarid southwestern United
<br />States, channels generally have incised their beds
<br />or degraded since the end of the 19th century
<br />(Graf, 1988).
<br />Regardless of the cause, if aggradation or
<br />degradation occurs in the control reach for a
<br />
<br />Mechanisms of Channel Change 3
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