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Figure 2. Aerial photographs of the debris fan at Lava Falls Rapid. A, (March 24. 1996) Lava Falls Rapid was constricted by a 1995 debris flow from Prospect Canyon. B. (April 9, 1996) Reworking during the rising limb of the 1996 controlled flood removed 5.900 m3 of the edge of the fan, increasing the width of the rapid by an average of 5 m. channel change is highly dependent on current efforts by the Grand Canyon Moniloring and Research Center (0 develop a baseline topographic map of the entire river channel. MOSI debris flows in Grand Canyon occur during the summt:r monsoon. Given that reworking can be subslanLial during high flows in lhe river, documenting new debris flows is best done annually between fall and early spring, Despite the large number of tributaries in Grand Can- yon. debris flows are relatively infre- quent; no more than 8 dehris flows haw been documented in any given year dur- ing the past decade (Melis and olhers. 1994; Webb and others. 2000). For pur- poses of comparison and maximizing Table 1. Types and accuracies of techniques for measuring debris-fan geometry bpected Horizontal Vertical Spacing Technique Frequency of Accuracy Accuracy Measuremen( (m) (m) (m) Survey On demand -0.01* -0.01' Variable (annual) BalhymelI)' On demand 0.05 0.05-0.06 Variable Digilal aerial Annual 1-5 ,,, 0.18 n.3. photos LTDA R Annual 0.30 0.15 2 Aerial photos Annual -1-]0" ". Variable n.a. f Depends upon instrumenl se[Up and rOOman accuracy. .. Depends upon the qualily of control points and the: camera and flighl characteristics. U. Topography can not be extracted without stereo phorography and comrol pands. information comenl. all monilOring should be done al rivCo:r discharges that are as equivalenl and as low as possible. Flow from the dam typically is low in the fall and early spring when h~a(ing and cool- ing demands for elecLIicilY are low. Several options exisl for measuring debris-fan volume and area (Table I). The most accurale measuremenl is oblained by combining direct survey of subaerial fan topography with multi-beam bathy- melfic measurements of the subaqueous debris fan. This is also the most expensive melhod. both in lenns of held-work and data processing. and data for the topogra- phy of the debris fan before the debris flow are seldom available. Remote-sensing techniques can over- come these limitations at (he expense of lower resolution and accuracy. The mosl promising technique uses image analysis of digital aerial photography. [f high-gain digital aerial photography is laken over clear. non-turbulent waler. the images can be analyzed 10 reveal subaqueous topog- raphy in shallow water. Digilal topogra- phy combining subaerial and subaqueous features can be developed from stereo images. The lechnique is new (Slaned in 2000) and cannol be used retrospectively for pre-debris now conditions. A slighLly less accurale bur time-sav- ing alternative is the use of LJDAR (Light Detection And Ranging). an airborne laser device which is ex.pected (0 be nown annually for the entire river corridor beginning in 2001. Th< accuracy of LJDAR data as collected in Grand Can- yon is considerably less man the diameter of mOSI of the coarse particles being mon- itored. LJDAR topography must be com- bined with either field assessmenlS or aerial photography to accura(ely map debris-fan boundaries and with multi- beam bathymetric dala 10 calculate a com- plele fan volume. Aerial mapping photography. which lypically is flown annually at a discharge of 8.000 f(J/s. is the least accurate bu( cheapest ~md most widely available (ech- nique. Aerial photography suitable for debris. fan monitoring has heen flown at least annually since the mid-1980s, pro- viding an excellenL baseline of pre-flow fan conditions. Comparison of pre- and post-event photography greatly aids in delineating the boundaries of debns flows ll" well as reworking by (he Colorado - fl L> )