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<br />discharge. The rate of sand accumulation as well as movement of sand within an eddy increase
<br />with increasing discharge. Thus, in general, a larger discharge will deposit and form eddy sand
<br />bars more quickly than a small discharge, In particular, deposition rate in an eddy depends
<br />directly on the concentration of sand in the main channel (Schmidt et aI., 1993) which varies by
<br />approximately the second to third power of the discharge. Thus, for a fixed volume of water, a
<br />larger flood for a shorter duration will likely build larger eddy sand bars, The larger discharge
<br />will also deposit sand at higher stages aoove the levels of normal power-plant release,
<br />For a given volume of water, a larger discharge for a shorter duration will transport more
<br />sand out of Grand Canyon and thus deplete a greater volume of the available supply stored on
<br />the bed. For this reason, estimates of the quantity of sand stored in the channel will primarily
<br />determine how frequently a specific flood can occur. Measurements of the rate of post-flood
<br />erosion and vegetation encroachment demonstrate that the high flood attributes of most sand
<br />deposits are obscured in less than 7 yrs (Kearsely et al., in press), oftentimes within 1-3 years.
<br />The optimum flood magnitude and duration for reestablishment of open,elevated bars should be
<br />such that the flood can be generated as frequently as possible without depleting the available
<br />supply of sand.
<br />Channel and river characteristics, such as debris-fan height, sand storage, and projected
<br />sand-transport load, affect determination of the optimum flood magnitude, duration, and
<br />frequency. One factor or another may be especially important at a given location. As described
<br />above, areas of separated flow are extinguished when river stage exceeds the top of the debris
<br />fan that causes a channel constriction. When areas of separated flow are extinguished, most if
<br />not all of the sand stored in the eddy is lost downstream. Consequently, the desired flood stage
<br />should be less than the height of debris fans located in critical reaches where campsite
<br />availability is limited (Kearsely et al., in press). Melis (T. S. Melis, hydrologist, 1993, U. S.
<br />Geological Survey, Tucson, written commtm.) has shown that these critical reaches are
<br />characterized by low-elevation debris fans. The optimization of flood magnitude, duration, and
<br />frequency thus will ultimately be related to the general characteristics of critical reaches targeted
<br />for management Presently, the characteristics of upper Marble Canyon seem to be most
<br />appropriate for detailed flood design.
<br />The proposed flood magnitude and duration are estimated based primarily on
<br />observations made at discharges less than power-plant capacity, plus some data collected
<br />between 1983 and 1986. It would be fortuitous if the proposed experimental flood was, in fact,
<br />optimum. Accordingly, purposes are three-fold First, to evaluate effects of this specific event
<br />Second, and perhaps most importantly, to gain a quantitative description and understanding of
<br />the essential proc~ses controlling (1) transport of sand in the main channel and (2) the
<br />temporary storage of sand in eddies and development and/or maintenance of return channels
<br />(often called backwaters for biological purposes). A detailed understanding of these proctSses
<br />will provide information needed to predict water and sediment flow through Glen, Marble and
<br />Grand Canyons and the rate of sand deposition in eddies at discharges not specifically measured,
<br />in addition to effects on riverine ecosystems. These support the third purpose which is to
<br />develop tools for more accurate evaluation and management of future spills.
<br />Whatever the outcome of this one experiment, we do not anticipate that there will be a
<br />single optimum flood proposed for management of the whole Grand Canyon. Rather, the
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