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<br />tributary inputs. The relations that he has developed for visible-wavelength image data can now <br />be used to calibrate airborne sensors to produce maps of these parameters throughout the CRE. <br />Relatively inexpensive ($15.000) digital camera systems (such as that used by Chavez) are <br />available that can record these required wavelengths, which would allow data collcction at any <br />time for the cost of aircraft and pilot time (about $650 per hour using a Bureau of Reclamation <br />(BOR) aircraft and pilot). This helicopter-mounted instrument can image about 100 miles ofthc <br />river corridor in two hours because the aircraft can follow the course of the corridor, but care <br />needs to be taken during turns so that the aircraft does not produce roll, which results in off-nadir <br />imagery. Although data collection is inexpensive. post-processing of these data without GPS and <br />IMU data will add to these costs. This factor is discussed in more detail in the next section. <br /> <br />4.1.2. Channel substrate <br /> <br />Channel substrate parameters consist of main-stem and tributary bathymetry, tine- vcrsus <br />coarse-grain-size distribution on beds, median grain-size distribution in vertical protiles of river <br />beds, and thickness of beds. Only the first two of these parameters can be approached using <br />airborne remote-sensing data. Bathymetry within the main stem is currently obtained using a <br />backscatter multibeam instrument that provides fine-scale (3-cm) topography, as recommended <br />by the physical resource PEP (Wohl et aI., 1999). Bathymetry of shallow near-shore <br />environments that cannot be surveyed with this instrument is measured by ground-survey crews. <br />Although the goal is to map the substrate bathymetry throughout the corridor every five years, <br />processing of the backscaner data is time consuming and the monitoring is behind schedule. <br /> <br />Alternative airborne remote-sensing techniques include the SHOALS UDAR and optical <br />image data. The physical resource PEP (Wahl et al.. 1999) recommended that SHOA LS <br />(Scanning Hydrographic Operational Airborne Lidar Survey) be considered for bathymetric <br />surveys. The SHOALS L1DAR system is a dual-beam laser system that obtains water depth by <br />differencing the distances recorded from the green wavelength laser (substrate) and the near- <br />infrared laser (water surface). Recent studies have shown that the combination of SHOALS <br />bathymetry and color aerial photography can greatly assist in the mapping of coastal substrate and <br />coral reefs (Chavez and Field. 2000a, 2000b; Chavez et al.. 2000a, 2000b), but those waters are <br />relatively clear. The water penetration of SHOALS is constrained by turbidity and. therefore, <br />will have limited application within the CRE (Irish and Lillycrop, 1999). The SHOALS system <br />was recently improved under Navy contract and is now called CHARTS (Compact Hydrographic <br />Airborne Rapid Total Survey). The CHARTS system provides a 2-m spot spacing. can penetrate <br />down to 50 m in clear water and as much as 20 m in turbid water (Heslin et al.. 2003). This <br />system may provide lotal channel geomelry in a relatively short time frame and should be tested <br />as a replacement for the multibeam backscatter system to map channel topography. <br /> <br />Two optical-image approaches for mapping bathymetry derive (I) relative water depth <br />from images acquired at two wavelengths and (2) absolute water depth from stereo-image pairs. <br />The first technique has been used in a variety of clean, standing water bodies (Lyzenga. 1978, <br />1981; Bagheri et al.. 1998; Bryant and Gilvear, 1999; Roberts and Anderson, 1999; Woodruffet <br />aI., 1999; Durand et a., 2000). This technique requires two wavelength bands because retlectance <br />from thc substrate can change with the substrate composition and images of two different <br />wavelengths can be used to separate and map water depth and bonom composition. as long as <br />both wavelength signals are retlected from the substrate. The maximum water depth that can be <br />determined using this method is limited by the maximum penetration depth of light in the longest <br />wavelength region and by the optical properties of the water. The suspended sediment within the <br />Colorado River will limit the application of this technique to a very small fraction of the CRE that <br />generally has clear water. Sun glint from rapids will create problems in this approach for <br /> <br />20 <br />