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CADDIS Volume 2: Sources, Stressors & Responses


You may encounter a variety of existing sediment-related data expressed in units or types of measurements listed below [see Edwards and Glysson (1998) for field methods for measurement of fluvial sediment]. In addition, measures of channel structure are included here because they can be informative regarding sources and mechanisms that alter sediment supply.

Ways to measure suspended sediment

Turbidity: The amount of light transmission due to absorption and scattering as affected by suspended sediments [nephelometric turbidity units (NTU)].

Total suspended solids (TSS, also termed total filterable solids): Suspended organic and inorganic solids that are not in solution and which can be removed by filtration (mg/L).

Suspended sediment concentration (SSC): Dry weight of sediment from a known volume of water-sediment mixture (clay, silt, sand, and organic matter) (mg/L).

Light penetration: Amount of light that can reach various depths of water due to attenuation (Secchi depth (m) or extinction coefficient).

Water clarity: Qualitatively reported observations of transparency of water.

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Ways to measure deposited and bedded sediment

Measures of DBS

Bedload sediment/bedload transport: Proportion of total sediment rolling, sliding, and bouncing along the stream bottom and being transported downstream. The proportion of bottom sediments moving as bedload will depend on, at a minimum, stream power, the sizes and size distribution of the available bottom sediment particles, the size and quantity of woody debris in the streambed, and the riffle/pool structure of the stream. Bedload is measured several ways and is usually expressed in kg/day (see U.S. EPA 2006b).

Percent fine sediment at surface: Proportion of fine sediment on substrate surface (percent fines at surface, % fines).

Percent fine sediment at depth: Proportion of fine sediment to a certain depth of substrate (percent fines at x cm of depth).

Sedimentation rate: Amount of suspended sediment that settles onto substrate per unit time, typically reported as grams per square meter of substrate per day (g/m2/d).

Embeddedness: Degree to which interstitial spaces between particles in coarse substrates are filled by finer particles (% embeddedness).

Suspendable solids: Amount of fine sediment re-suspended upon disturbance of streambed, usually described in grams per square meter of substrate (g/m2).

Settleable solids: Essentially the same as suspendable solids, equal to amount of fine sediment suspended during disturbance of streambed (typically during periods of increased stream discharge) and subsequently re-settling onto streambed as disturbance subsides or high discharges return to baseflow. Often reported as grams of sediment per square meter of substrate (g/m2).

Particle size distribution: Relative proportion of different particle sizes comprising the stream bed, often summarized as the fraction of the substrate in boulders, cobbles, gravel, sands, and silt/clay (e.g., % silt, % cobble).

Particle size geometric mean: Geometric mean of the particle sizes (mm, as measured from intermediate or median particle axis diameters).

Median particle size: Commonly know as D50, particle size at the 50th percentile of bed material size distribution (mm, as measured from intermediate or median particle axis diameters).

Substrate stability: Ease with which deposited sediments may become re-suspended, often described as grams of sediment re-suspended within a unit area (m2) of the streambed at a given level of stream power (in watts), (g/m2/W).

Relative bed stability: Index of substrate mobility, equal to the ratio of the particle size of observed sediments to the size of sediments that each stream can move or scour during its flood or bankfull stage (mm/mm ratio).

Bottom deposit depth: Depth of fine sediments covering the streambed (mm or cm).

Pebble count: A description of the particle size distribution of the streambed sediment particles obtained by measuring the length of the intermediate/median axis of approximately 100 randomly-chosen streambed particles. Particle selection is typically done while walking in the stream in multiple cross-sectional transects or while crossing the stream and moving upstream in a zig-zag pattern. Once 100 particles have been measured, a cumulative particle size distribution is graphed to determine median substrate particle size, as well as other percentiles in the substrate particle size distribution. See Wolman (1954) for further details.

Measures of channel structure related to sources and indirect measures of sediment

Residual pool volume: Volume of water in pools at low flow.

Bank stability: A measure of the susceptibility of a stream bank to erosion.

Waterbody dimensions: Measurements of width, mean depth, and depth at thalweg.

Bathymetry: Spatial variation in water depth (i.e., underwater topography).

Riffle/pool ratio: Proportion of stream channel length in riffles versus pools.

Gradient: Slope over which a stream flows, or the change in elevation (“rise”, in meters) divided by the distance over which this change in elevation occurs (“run”, also in meters); no units in calculated slope as they cancel out.

Sinuosity: Degree to which a stream meanders, measured as actual stream channel length divided by the straight-line distance between the starting and ending points (m/m or km/km).

Incision Downcutting of a stream bed, usually measured as the increase in depth over time (e.g., m/y).

Bank erosion: Visible loss of soil and vegetation from stream banks, may be measured as horizontal bank loss over time (e.g., cm/y).

Channel braiding: Extent to which a streambed is divided into multiple channelsThe term “anastomosing channels” usually refers to reaches with relatively long major and minor channels branching and rejoining in a complex network. Stream reaches classified as “braided channels” also have multiple branching and rejoining channels, but these sub-channels are generally smaller, shorter, and more numerous, often with no obvious dominant channel(s).

Ways to measure insufficient sediment

Ways to measure deposited and bedded sediment also apply to insufficient sediment.

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