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<br />NAU Sand Bar Studies
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<br />FilUll Report
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<br />left that we refer to as Eddy 2, We surveyed 9,500 m2 of this eddy. Segments 2 and 3 are bordered by
<br />
<br />
<br />small eddies and channel-margin deposits on the inside of bends. The downstream end of Segment 3
<br />
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<br />is a small riffle formed by a low debris fan and rock fall. Segment 4 includes an eddy along the left
<br />
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<br />bank that we referto as Eddy 3, We surveyed 6,100 m2 of Eddy 3. This eddy was fIrst surveyed in
<br />
<br />
<br />1985 (Schmidt and Graf, 1990), and the eddy and adjacent channel have been surveyed one or more
<br />
<br />
<br />times a year since 1991 (Hazel ei al" 1999, site #3). Sand has been subaerially exposed at some time
<br />
<br />
<br />in Eddies 1,2, and 3 between 1935, and the present (H, Sheik, pers, comm" 1999),
<br />
<br />
<br />Hydrographic surveys of the entire reach were collected before and after the first, and after the last
<br />
<br />
<br />of the Paria River floods in 1997, and daily hydrographic and topographic surveys were conducted in
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<br />
<br />Segments 3 and 4 for 4 days before, during, and after the 1997 Test Flow (Table 1), During the 1997
<br />
<br />
<br />Test Flow, Segment 3 was surveyed once a day and Segment 4 was surveyed twice a day, With the
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<br />
<br />exception of Eddy 3 where survey coverage extends to the area inundated by flows of 1,274 m3ts
<br />
<br />
<br />(45,000 ft3tS), subaerial bank and bar deposits higher than the elevation reached by flows of 566 m3ts
<br />
<br />
<br />(20,000 ft3ts) or in areas that extend beyond our survey limits were not examined in the 3-km reach,
<br />
<br />
<br />Ground and hydrographic points were combined and topographic surface models created using the
<br />
<br />
<br />triangulated irregular network method of contouring with surface modeling software, To compare the
<br />
<br />relative proportion of sand stored in pools and in eddies, area and volume calculations were
<br />
<br />
<br />differentiated for the two environments by utilizing a boundary that estimates the position of the eddy
<br />
<br />
<br />fence, the streamline dividing downstream flow and the eddy, and by assuming this zone extends
<br />
<br />
<br />vertically to the bed (Fig, I). It is important to note that eddies change in length with changes in flow
<br />
<br />
<br />(Schmidt, 1990), In our analysis, eddy fence location was determined by aerial photographs and by
<br />
<br />
<br />surveying the positions of separation and reattachment points in the fIeld at different discharges, This
<br />
<br />
<br />general approximation of eddy-fence location best represents the eddy dimensions at most flows within
<br />
<br />
<br />1,274 m3ts (45,000 tt3ts), Accuracy and precision of these techniques are discussed in Beus et al,
<br />
<br />
<br />(1992), Andrews et al, (1999), and Hazel et al, (1999), Area and volume calculations were rounded to
<br />
<br />
<br />reflect the accuracy of ground and hydrographic points, Conversions of sand volumes to mass were
<br />
<br />
<br />made assuming a porosity of 35% and a bulk density of 2,65 Kg/m3 for sand-sized sediment.
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<br />Field Studies oJ Sites in Marble Canyon
<br />Annual and more frequent surveys of thirty-one to thirty-five long-term study sites located in the
<br />Colorado River ecosystem are reported for data collected between 1991 and 1998, by Kaplinski et ai,
<br />(1995; 1998) and Hazel et ai, (1999), Sand bar and subaqueous channel bed change were measured
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<br />13
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