JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION VOL. 38, NO. 4 AMERICAN WATER RESOURCES ASSOCIATION AUGUST 2002 A RIFFLE STABILITY INDEX TO EVALUATE SEDIMENT LOADING TO STREAMS1 Gary B. Kappesser2 ABSTRACT: Riffles in moderately entrenched stream and biological integrity of the Nation's waters is being reaches with gradients of 2 percent to 4 percent that implemented through watershed assessments at vari- have received excessive sediment from upstream have ous scales. Although broad scale condition and trend a distinctly different and higher proportion of smaller assessments can be done using indirect indicators mobile particles than riffles in systems that are in such as USEPA's BASINS model, models cannot sub- dynamic equilibrium. The mobile fraction on the riffle stitute for long term monitoring (Halpern, 2000). One can be estimated by comparing the relative abun- of the more elusive parameters to quantify is dance of various particle sizes present on the riffle increased sediment supply to streams. Erosion and with the dominant large particles on an adjacent bar. deposition can be increased profoundly by human- Riffle particles smaller than the dominant large parti- caused disturbance in a watershed. Increases in sedi- cles on the bar are interpreted as mobile. The mobile ment deposition can result in adverse effects to percentile of particles on the riffle is termed “Riffle beneficial uses of mountain streams by reducing habi- Stability Index” (RSI) and provides a useful estimate tat complexity and quality for cold water fisheries of the degree of increased sediment supply to riffles in (Hicks et al., 1991). mountain streams. The RSI addresses situations in This paper presents a Riffle Stability Index (RSI) which increases in gravel bedload from headwaters method for measuring the response of sediment trans- activities is depositing material on riffles and filling port channel types (Schumm, 1977) to increases in pools, and it reflects qualitative differences between sediment supply. The strategy of the methodology is reference and managed watersheds. The RSI corre- to determine the particle size percentile of the riffle lates well with other measures of stream channel that is mobile and use it as an index of sediment physical condition, such as V* and the results of fish input from upstream. RSI data collected from man- habitat surveys. Thus, it can be used as an indicator aged and reference watersheds in Idaho and Virginia of stream reach and watershed condition and also of are compared to demonstrate the sensitivity and utili- aquatic habitat quality. ty of the technique for interpreting watershed condi- (KEY TERMS: bedload transport; sedimentation; peb- tion in these streams. RSI is also compared with other ble count; riffles; channel stability; monitoring.) methods for evaluating stream channel physical con- dition to validate the RSI methodology. INTRODUCTION THEORETICAL BACKGROUND In recent years there has been increasing interest in stream and watershed condition. For example, the A stable stream reach is one in dynamic equilibri- broad goals of the Clean Water Act (33 U.S.C. 1251- um. Over a time frame of several years, sediment size 1387) to restore and maintain the chemical, physical, and sediment transport rates into a reach are similar 1Paper No. 01174 of the Journal of the American Water Resources Association. Discussions are open until February 1, 2003. 2Hydrologist, USDA Forest Service, George Washington and Jefferson National Forests, 5162 Valley Pointe Parkway, Roanoke, Virginia 24019 (E-Mail: [email protected]). JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION 1069 JAWRA Kappesser to those exiting the reach (Lane, 1954; Heede, 1980). depth ratio greater than 12, and moderate sinuosity When sediment transport rates into a reach exceed (greater than 1.2). B3 and F3b stream reaches have those exiting, some sediment is deposited within the streambeds that are predominantly cobble with lesser reach. Accelerated deposition is typically accompanied amounts of boulders, gravel, and sand (Rosgen, 1996). by a textural change of the bed material (Lisle, 1982; Within the range of cold water fish in national forests, Madej, 1982). If the imbalance persists, channels may B and Fb channels represent some of the most impor- widen; pools may shorten, become less deep, and tant habitat (Dave Cross, personal communication). transform into runs (Bisson and Sedell, 1982); and Headwaters of natural gravel bed rivers have many general aggradation of the bed surface may occur grain sizes, some of which can be transported down- (Lisle, 1982). Of these responses, textural change, stream under commonly occurring flows and some defined as a change in the relative abundance of par- which cannot except under extremely high flows or ticles of different sizes on the streambed, was found floods (Gessler, 1971). During flow events capable of by Lisle et al. (1993) to be the only hydraulic variable transporting bedload, sediment in stream systems in that responds consistently to changes in sediment dynamic equilibrium is scoured from pools and load. deposited on riffles (Hirsch and Abrahams, 1981; Streambed texture can be estimated using Wolman Lisle, 1979). As flow decreases, finer particles are pebble counts (Wolman, 1954). In the pebble count winnowed out of the riffles and deposited in the next procedure, at least 100 particles are picked from the pool downstream (Hirsch and Abrahams, 1981). bed on the basis of a grid system, the intermediate When sediment supply from upstream exceeds the axis is measured, and a frequency distribution is stream's competence to transport it, pools fill and rif- developed. Sampling is best conducted within wad- fles load with finer mobile sediment (Andrews, 1982). able areas of the channel that have homogeneous pop- Riffles represent concentrations of larger residual ulations of substrate (Kondolf, 1997), such as riffles or particles in stream channels (Yang, 1971; Church and pools. In riverbeds with poorly sorted bed material, a Jones, 1982) and therefore offer the greatest contrast greater number of pebbles may be needed to ade- between residual particles and those that are mobile. quately capture the greater variability (Kondolf, 1997; Thus, riffles offer logical sites to determine channel Wolman, 1954). bed texture. Beschta (1987) presented a conceptual model of To use riffle substrate as an index of streambed sediment transport in streams in which most streams condition, the size of the largest bedload particles function as integrated systems and where processes mobile during frequent flood events must be known. in individual reaches influence those in other reaches. Point bars have been identified as sources of this For example, during periods of accelerated headwater information. Point bars are composed of and formed erosion, large amounts of sediment become available by the coarse part of the total bedload (Leopold, 1992, for downstream transport. Generally, headwater 1994), which is deposited primarily during near flood streams have steep gradients and can route sediment stages (Bridge and Jarvis, 1982). Rosgen (1996) found rapidly to downstream reaches. As this sediment is that core samples from point bars and central bars routed downstream, it encounters decreasing channel provide good indicators of bedload sizes that are gradients where deposition, particularly of coarser transported at normal high flows. grained materials, occurs. Bar development usually occurs only in channels In many mountainous regions of the United States, with gradients of approximately less than 5 percent; deposition of coarse grained bedload material is likely above that gradient bars rarely form (Church and to occur in stream reaches that classify as B and Fb Jones, 1982). Classic point bars are found in low gra- channel types (Rosgen, 1996). These Rosgen B3 and dient meandering streams with higher sinuosity (e.g., F3b channels are nearly identical in appearance, geo- sinuosity > 1.2). Smaller, less extensive bar deposits morphology, function, and fish utilization. However, called lateral bars are found in higher gradient, more the entrenchment ratios of the F3b channels are less entrenched streams (Church and Jones, 1982) such as than 1.4; whereas the B3 channels have entrench- B or Fb type streams (Rosgen, 1996). Lateral bars are ment ratios between 1.4 and 2.2. Entrenchment ratio similar to point bars in that they contain deposits of is defined as the ratio of the width of the flood prone the larger sizes of material transported as bedload. area to the surface width of the bankfull channel. The Therefore, a sample of the largest particles on a later- flood prone area width is measured at the elevation al bar can be used to estimate the largest particles that corresponds to twice the maximum depth of the mobilized during high flow events. The largest parti- bankfull channel as taken from the established bank- cles on a bar can be identified by systematic selection, full stage (Rosgen, 1996). Both channel types have a as well as by random sampling (Bluck, 1982; Ash- median particle size (D50) of 64 to 256 millimeters, worth and Ferguson, 1989). To characterize bar channel gradient between 2 and 4 percent, width/ deposits, Bluck (1982) selected 30 of the largest clasts JAWRA 1070 JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION A Riffle Stability Index to Evaluate Sediment Loading to Streams from bar deposits, and Ashworth and Ferguson (1989) watersheds have visible attributes of stability as iden- measured the mean diameter of the 10 largest parti- tified by Pfankuch (1975). Managed watersheds on cles on the barhead. The geometric mean of the 10 to the IPNF commonly had large clear cuts, sometimes 30 largest particle sizes on a bar, termed the domi- extending for thousands of acres. A high density of nant bar particle size, can be compared with the logging roads was necessary to support the past har- cumulative size distribution of the range of particle vest. Typically, logging roads were constructed on a sizes present on an adjacent riffle to estimate the per- 122 m parallel spacing on the contour, or with a road centile of mobile riffle material.