Reconfigured Channel Monitoring and Assessment Program (RCMAP) Completed

The U.S. Geological Survey was engaged in a program to monitor and assess the long-term geomorphic behavior of selected river and stream reaches that previously had undergone some physical modification.  These modifications included natural channel adjustments to floods as well as intentional channel reconfigurations to alter the function or appearance of a river reach. 

RCMAP Objectives

The objectives of the USGS Reconfigured-Channel Monitoring and Assessment Program (RCMAP):

1. Develop uniform and versatile monitoring methods for reconfigured channel reaches and to apply these methods to selected reaches.

2. Create and maintain a data base consisting of numerous monumented stream reaches; and

3. Revisit these reaches periodically and assess regional and temporal trends in the geomorphic response of the stream to the channel modifications.

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Long-term monitoring of reconfigured channels should enable analysis of how and why a particular reconfiguration design may have remained stable or failed. If a channel modification fails, the analysis would focus on understanding the processes by which failure occurred. These processes could include bank erosion, streambed aggradation or incision, flood-plain deposition or scour, and loss of riparian vegetation through root scour, soil-moisture deficit, or prolonged submergence.

Study Areas

1. Muddy Creek below Wolford Mountain Reservoir
2. Roaring Fork River at Basalt
3. Gunnison River at Spann Diversion
4. Lake Fork at Gateview
5. North Fork Gunnison River at Hotchkiss
6. Uncompahgre River at Ridgway
7. Lake Fork of the Gunnison River near Lake City
8. Gunnison River at the Hartland Dam near Delta, Colorado

Future Activities

Channel adjustments are the expected behavior of alluvial rivers; however, the rate of channel adjustment can range from imperceptible to dramatic and can affect river function and water-resource utilization. The recently monumented channels will be resurveyed and rephotographed, and the sediment will be recharacterized in the future. Replicate measurements will be made to quantify changes in channel geometry and sediment-size characteristics and to determine how and why a particular reconfiguration design may have remained stable or failed. The replication interval will be determined largely by year-to-year streamflow characteristics (recurrence of floods) and the presence or absence of geomorphic adjustment.

Channel modification and reconfiguration projects have been considered for many other river and stream reaches in the Western United States. The RCMAP included surveys of other recently reconfigured stream reaches and will revisit previously monumented reaches as opportunities arise. 

Monitoring Methods

Site specific activities of the Reconfigured Channel Monitoring and Assessment Program consist primarily of descriptive measurements of physical channel characteristics prior to and following channel modification. These activities also might include geomorphic, hydrologic, and biologic evaluations of the river reach. On-site measurements are tailored to a specific stream reach and to the reclamation objective. Measurements include surveys of the channel cross section and longitudinal profile, measurement of sediment-size characteristics of the streambed and banks, and oblique photography from monumented locations through the reach. Because these surveys, measurements, and photographs are to be replicated, it is essential that they be referenced to permanent monuments and that standardized methods be employed.

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The aerial extent of monitoring is dependent on the size of the reconfiguration project, geomorphic and ecological variability within the reconfigured reach, the project budget, and other site-specific considerations. At a minimum, monitoring data are collected over a representative stream reach of at least several channel widths in length. Multiple, discontinuous monitoring reaches may be established if the reconfiguration project is large.

A set of measurements are made prior to reconfiguration, if possible, and during the first year after reconfiguration. Measurements in an upstream and/or downstream control reach (where reconfiguration has not been undertaken) is desirable. These measurements will be replicated in subsequent years to evaluate channel change in the reconfigured reach. The interval between replicate measurements will be determined partly by the hydrologic history and geomorphic response of the monitored reach.

Channel Surveys

A survey of the stream channel is a critical component of the monitoring activities and provides a reference for other measurements and photographs. Replicate channel surveys with a common datum and coordinate system will enable detection of geomorphic change that might occur as a result of flood scour, bed-material aggradation, or lateral channel migration. Standard geomorphic field techniques are used in RCMAP monitoring activities. The USDA Forest Service General Technical Report RM-245 provides guidelines for basic surveying techniques, identification of bankfull indicators, and measuring other important stream characteristics.

A total-station survey or a survey-grade GPS survey is recommended for channel monitoring. The channel survey must produce location (Northing and Easting) and elevation data that can be used to quantify longitudinal and cross-sectional characteristics of the reach (fig. 2). These data also can be used to generate topographic maps of the reconfigured reach, the streambed, gravel bars, banks, and low terraces. Miscellaneous features, such as levees, diversion structures, habitat-enhancement boulders, and bank-protection structures also can be surveyed.

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The survey should be referenced to a datum and coordinate system that can be reoccupied by subsequent surveys. If a local datum and coordinate system are used, the location of reference marks should be identified with Latitude and Longitude coordinates (decimal degrees) determined from global-position satellite (GPS) methods. The GPS datum used must be identified. The USGS recommends use of the NAR-83 datum for GPS measurements. Survey monuments can include rebar or brass caps set in concrete, chiseled marks in bedrock or concrete structures, bolts on bridges, or other identifiable items not susceptible to frost heave, vandalism, and flood or animal damage. Multiple reference points are desirable. Vertical accuracy of 0.01 ft is required for all monuments and reference points; accuracy of 0.1 ft is acceptable for ground points. The scope of the channel survey varies but could include the river bed, low-lying gravel and cobble bars, islands, side channels, banks, levees, and nearby terraces that would contain up to about the 5- or 10-year flood. One purpose of the channel survey is to capture the topographic variability of the streambed and nearby surfaces that the water may flow over. Vertically, the survey should include the deepest portion of the channel to the top of the flood-confining terrace, bank, or levee. Where the channel cannot be waded safely, estimates of streambed elevation can be made from direct observation, but should be clearly labeled as estimates.

Cross Sections

Channel cross sections should be physically surveyed on site and not "cut" or extracted from the topography generated by mapping software. Streambed detail for monitoring purposes is not adequately represented in a cut or extracted cross section. Monitoring cross sections are most beneficial if located along the reconfigured channel in areas most sensitive to alteration by streamflow; for example, in areas that might scour or aggrade as bed material is transported, or in meander bends where lateral erosion could occur. Cross sections should be oriented perpendicular to the bankfull flow and should be spaced up- and downstream along the channel at an average of about 3 to 6 times the mean bankfull channel width (fig. 2). In longer reconfigured reaches, the monitoring cross sections can be clustered in shorter subreaches that are representative of the whole reach. Cross section spacing is determined by site-specific considerations. Cross sections can be spaced wider apart where the channel is uniform (has little curvature, similar cross-section shape, same grade, same roughness) and should be spaced more closely where the channel is irregular (width or slope vary abruptly, islands or bends are present, roughness varies), near bridge abutments and piers, and near flow-directing structures commonly used in channel reconfiguration. If streamflow modeling is proposed, the cross sections should be spaced more closely to capture downstream variations in streambed topography. Survey shots along the cross section should be in a straight line (under a stretched tag line or along a navigated course between two GPS-located endpoints), and should number about 30-40 locations between the banks. Closer spacing is required when there is great streambed irregularity, wider spacing is acceptable when the streambed is more uniform and on the flood plain.

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Cross-section end point monuments can be rebar, a T-post, or some other item that will not be altered by an attached tagline because the endpoint monument also is the local cross-section reference for northing, easting, stationing and elevation. If there is a possibility that an attached tagline will compromise a cross section endpoint, then a separate tagline anchor should be installed. Ideally, the endpoint monuments are located close enough to the river (on a bank or terrace) so that it is convenient to attach a tagline, and high enough to be safe from flood scour or debris; this often is a compromise.

Longitudinal Transect Surveys

Longitudinal channel transects define the downstream slope or grade, and identify, for mapping purposes, the edge of water, bars, banks, and other topographic features. These transects could include the left and right edge of water, the deepest (thalweg) part of the channel, the left and right banks, the tops of gravel bars, levees, and terrace scarps. Features surveyed will vary from reach to reach. Longitudinal transects do not need endpoint monuments because their location will change from year to year in future resurveys and because shots along them are referenced to the channel survey northing and easting coordinates. Spacing between longitudinal transect shots can be wider than for the cross section shots.

Survey Point Numbers and Descriptive Codes

Each survey shot should have a unique point number. Descriptive codes are desirable but not mandatory for all shots. Descriptive codes should be created for all bench marks, reference points, photo monuments, cross section end points, or other permanent monuments. Other descriptive codes might include: a cross-section identifier, the edge of water, high-water marks, top of bank, estimated sediment size (sand, gravel, cobbles), cobble bar, gravel bar, toe of bank or slope, riffle, pool, change in vegetation, flow-directing structures (vortex weir, cross vane), levee, riprap, gabions, and bridge abutments or piers, etc. There is great room for creativity with descriptive codes. Point numbers and descriptive codes will help subsequent investigators reproduce and interpret the surveys.

Monumented Photographs

Oblique photography from monumented locations compliments the channel survey and sediment measurements. The monumented photographs include channel features not specifically included in the surveys as well as information about sediment deposits and riparian vegetation. When replicated, these photographs are very useful in documenting qualitative changes in the monitored reach. The photographs initially can be made with a variety of photographic devices, e.g., single-lens reflex camera, digital camera, slide, print or digital image. However, to be most useful, the photographs should be taken from monumented locations that can be re-occupied in subsequent years, and the location and specific details of the photographs must be recorded. Photographic monuments can be cross section endpoints, survey reference marks, a chiseled boulder on the streambank, a marked location on a bridge, or some other permanent reference mark. These monuments should be included in the channel survey and described in the field notes. The photographs should be oriented to show a variety of relevant features, such as the streambanks, channel curvature, bank and bar sediment, flow-diverting structures, riffles, pools, and other characteristics. Usually, several photographs are required to document the monitoring reach. Date, time of day, and view orientation (east, west, upstream, downstream, etc.) must be recorded. The approximate camera height above the reference point (standing, sitting) and the camera focal length (28 mm, 50, mm, etc.) also must be recorded so that replicate photographs can be made with minimal parallax error. Identification in the field notes of the photo subject is helpful, for example, "left bank near cross section 7", or "boulder weir 15 feet downstream from cross section 3". An estimate of the discharge also is informative because variations in water-surface stage can affect visual perception of channel features.

Sediment Measurements

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Alluvial stream channels are composed of sediment that periodically may become entrained. Flow induced movement and redeposition of fluvial sediment, as well as the sediment size and sorting, have an effect on the hydraulic roughness, streamflow conveyance, and aquatic habitat of the stream. Quantification of sediment size-characteristics is an important component of reconfigured channel monitoring. Sediment characteristics are determined for a variety of geomorphic surfaces in the monitoring reach. These include the streambed, alluvial bars, and the streambanks. The objective of the alluvial bar and bank sediment measurements is to characterize discrete geomorphic surfaces, and not to average these features into a single measurement. The objective of the streambed measurement is to characterize the range of sediment particles available for transport within the channel. The streambed measurement can include some bar and bank material, but predominantly is composed of sediment inundated by less-than bankfull streamflows. When the sediment is gravel-size or larger, measurements are made with the Wolman pebble count method (Wolman 1954). When the sediment is predominantly fine gravel, sand, or finer, samples are collected on-site and sieved in the laboratory. Streambed sediment measurements are usually made linearly across the low-discharge channel from bank to bank. Alluvial bar and bank sediment measurements are usually made linearly and parallel to the direction of streamflow (fig. 1). In some situations, a non-linear, random path sample is made. Sediment particles are selected for measurement at intervals greater than the largest particle diameter on the surface to be sampled. Usually, one hundred measurements provides a statistically representative sample of a geomorphically unique surface; however, more measurements may be required if the particle size is highly variable on the surface. The intermediate, or "b-axis" of the sediment particle is measured with a graduated scale to the nearest millimeter for gravel and small cobbles, and to the nearest 5 millimeters for large cobbles and boulders. The precise b-axis length is recorded in the field notes and is preferred to noting an equivalent sieve-size range, for example, 45 to 64 mm. Size statistics are computed from the cumulative-frequency distribution function of the intermediate-axis measurements and include the median particle size (the size at the 50th percentile, or d₅₀), and sizes at other percentiles, such as the 16th and 84th.