Prepared in cooperation with the Grand Canyon Monitoring and Research Center Monitoring of Coarse Sediment Inputs to the Colorado River in Grand Canyon Introduction Debris flows can have an immediate rapids, potentially altering the eddy pat- and dramatic effect on the river corri- tern and increasing the length of the rapid. Coarse sediment (particles with an dor. Even a single small debris flow As a result, debris fans and rapids may be intermediate diameter > 64 mm) affects may significantly alter the topography aggrading over the long term. the primary components of the Colorado and hydraulics of a debris fan and rapid River ecosystem. The deposition of in a matter of minutes. However, the Monitoring Debris Fans coarse sediment at tributary junctures Colorado River redistributes the coarse builds large debris fans that constrict the sediment introduced by debris flows The effective monitoring of coarse river and form rapids (fig. 1). Debris fans, almost immediately after deposition sediment requires both the short-term and the debris bars that develop below and during subsequent high flows. documentation of inputs by debris flow rapids, provide stable substrate for aquatic Before closure of Glen Canyon Dam, and the long-term evaluation of the redis- organisms, notably the alga Cladophora large floods on the river routinely tribution of that sediment by the Colorado glomerata. The pool above and recirculat- removed all fine sediment and some River. Both efforts involve measuring the ing eddy below the debris fan effectively coarse sediment from aggraded debris volume and particle-size distribution of trap fine sediment for storage on the bed fans (a process called reworking), trans- sediment delivered, as well as the effects or in sand bars. Debris fans and debris porting coarse sediment through the of its redistribution on the morphology bars form the fan-eddy complex that pool below the rapid and depositing it and hydraulics of the river channel. Moni- attracts humpback chub (Gila cypha), an as debris bars (fig. 1). In the regulated toring debris flows at regular intervals endangered species. Monitoring the input river, floods of reduced magnitude do will not only alert managers and research- of coarse sediment to the Colorado River not have sufficient stream power to ers to sudden, potentially important ecosystem and its long-term redistribution rework aggraded debris fans as thor- changes to channel resources but also will by the river is critical to the understanding oughly (Webb and others, 1999a, add to an existing database designed to and management of these valued 1999b). Coarse particles that are enable modeling of the interaction of resources. This fact sheet presents an entrained by these lower discharges coarse sediment and the Colorado River. overview of methods for monitoring may be deposited in the pools below The effective and efficient monitoring of coarse sediment input and redistribution in Grand Canyon. These methods are dis- cussed more thoroughly in Melis (1997), Melis and others (1994, 1997), and Webb and others (1999a, 1999b, 2000). Debris Flows and the River In small tributaries of the Colorado River between Powell and Mead reser- voirs, coarse sediment is transported to the river almost exclusively by debris flow. While tributary streamflow deposits are well-sorted and typically have less than 3% coarse sediment by weight, debris-flow deposits are poorly sorted and contain 5 to 76% coarse sediment (Webb and others, 2000). Figure 1. Schematic diagram of the fan-eddy complex on the Colorado River. U.S. Department of the Interior USGS Fact Sheet 019–01 U.S. Geological Survey February 2001 information content, all monitoring should be done at river discharges that are as equivalent and as low as possible. Flow from the dam typically is low in the fall and early spring when heating and cool- ing demands for electricity are low. Several options exist for measuring debris-fan volume and area (Table 1). The most accurate measurement is obtained by combining direct survey of subaerial fan topography with multi-beam bathy- metric measurements of the subaqueous debris fan. This is also the most expensive method, both in terms of field-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 the expense of lower resolution and accuracy. The most promising technique uses image analysis of digital aerial photography. If high-gain digital aerial photography is taken over clear, non-turbulent water, the images can be analyzed to reveal subaqueous topog- raphy in shallow water. Digital topogra- Figure 2. Aerial photographs of the debris fan at Lava Falls Rapid. A, (March 24, 1996) phy combining subaerial and subaqueous Lava Falls Rapid was constricted by a 1995 debris flow from Prospect Canyon. B, (April 3 features can be developed from stereo 9, 1996) Reworking during the rising limb of the 1996 controlled flood removed 5,900 m images. The technique is new (started in of the edge of the fan, increasing the width of the rapid by an average of 5 m. 2000) and cannot be used retrospectively channel change is highly dependent on debris flows is best done annually for pre-debris flow conditions. current efforts by the Grand Canyon between fall and early spring. Despite the A slightly less accurate but time-sav- Monitoring and Research Center to large number of tributaries in Grand Can- ing alternative is the use of LIDAR (LIght develop a baseline topographic map of the yon, debris flows are relatively infre- Detection And Ranging), an airborne laser entire river channel. quent; no more than 8 debris flows have device which is expected to be flown Most debris flows in Grand Canyon been documented in any given year dur- annually for the entire river corridor occur during the summer monsoon. Given ing the past decade (Melis and others, beginning in 2001. The accuracy of LIDAR data as collected in Grand Can- that reworking can be substantial during 1994; Webb and others, 2000). For pur- yon is considerably less than the diameter high flows in the river, documenting new poses of comparison and maximizing of most of the coarse particles being mon- Table 1. Types and accuracies of techniques for measuring debris-fan geometry itored. LIDAR topography must be com- bined with either field assessments or Expected Horizontal Vertical aerial photography to accurately map Spacing Technique Frequency of Accuracy Accuracy debris-fan boundaries and with multi- (m) Measurement (m) (m) beam bathymetric data to calculate a com- plete fan volume. Survey On demand ~0.01* ~0.01* Variable Aerial mapping photography, which (annual) typically is flown annually at a discharge Bathymetry On demand 0.05 0.05-0.06 Variable of 8,000 ft3/s, is the least accurate but *** cheapest and most widely available tech- Digital aerial Annual 1-5 n.a. 0.18 nique. Aerial photography suitable for photos debris-fan monitoring has been flown at LIDAR Annual 0.30 0.15 2 least annually since the mid-1980s, pro- viding an excellent baseline of pre-flow ** *** Aerial photos Annual ~1-10 n.a. Variable fan conditions. Comparison of pre- and * Depends upon instrument setup and rodman accuracy. post-event photography greatly aids in ** Depends upon the quality of control points and the camera and flight characteristics. delineating the boundaries of debris flows *** Topography can not be extracted without stereo photography and control panels. as well as reworking by the Colorado River (fig. 2). Standard aerial photographs ing, or the interlocking of particles. Sutur- Percent constriction of the river chan- must be digitized, georeferenced, and rec- ing is caused by the rearrangement and nel, a ratio of the average channel width tified before use, requiring the establish- wearing together of particles, and its through the rapid to the average channel ment of control points in the field. When occurrence makes debris fans difficult to width above and below the rapid (Webb pre-event topography is not available, fan rework. and others, 1999a), is a useful measure of volume can be calculated by multiplying the impact of a debris flow on river chan- Other measurements include survey- the fan area measured from aerial photo- nel morphology (fig. 5). For ease of mea- ing the water-surface fall through the graphs by an average thickness. surement and consistency, measures of rapid, which can be used to calculate Particle-size distributions are best channel width are best obtained from geo- stream power for a given discharge (fig. measured by combining point counts in referenced remote-sensing data, particu- 4). Surface velocity through the rapid can the field with standard sieve analysis in larly aerial photographs. Aerial be the laboratory to capture the full range of measured by timing the passage of photography may also reveal qualitative particles found in debris flow deposits. floats through the rapid. Other on-site changes in the hydraulics and navigability Larger particles may be measured on site. measurements include documentation of of rapids, reflecting underlying changes in Both unreworked deposits on fan surfaces changes in hydraulic features such as the the positions of boulders, and can be com- and reworked deposits along distal fan shifting of waves and holes, and the bined with field observations as a qualita- edges are evaluated (fig. 3). Particle-size movement, appearance, or disappearance tive measure of channel change. measurements should document sutur- of rocks. Because debris bars typically are unstable, much of the monitoring of these bars is best performed using remote sens- ing. This monitoring is most accurate when digital aerial photographs georefer- enced by geographical positioning sys- tems (GPS) are used because stable, long- term control points may be difficult to locate.