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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2008–5093

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Glossary

armor layer A coarser surficial layer of sediments on the streambed. This layer ranges from one particle thickness to several. This layer can be quite resistant to scour—usually only high flows mobilize this layer and it may re-form as flows decrease.

backwater Water backed up or retarded in its course as compared with its normal or free-flowing condition. Backwater is an increase in upstream flow depth due to a dam, constriction in a channel, change in channel slope, or change in roughness. Backwater is denoted by hydraulic engineers as an M₁ curve.

bedform Alluvial-channel bottom feature that depends on bed-material size, flow depth, and flow velocity. Bedforms include ripples, dunes, antidunes, and plane bed.

boundary or bed shear stress Stress that acts in the direction of flow on the streambed. Bed shear stress is often related to the ability of flow to move sediment on the streambed.

channel The channel includes the thalweg and streambed.

confluence The flowing together of two or more streams; the place where a tributary joins the main stream.

conveyance A measure of the carrying capacity of a channel section and is directly proportional to channel discharge. Conveyance is that part of Manning’s equation that excludes the square root of the energy gradient or friction slope.

critical flow The depth at which flow is at its minimum energy. Inertial forces equal gravitational forces.

cross section A series of coordinate pairs of elevation and stationing that describes the channel shape perpendicular to the mean flow direction.

flood Any relatively high streamflow that overtops the natural or artificial banks of a river.

floodplain Land adjoining (or near) the channel of a watercourse that is usually dry, but has been, or may be, covered by floodwaters. A floodplain functions as a temporary channel or reservoir for overbank flows.

free-flowing water Water that is not backed up or retarded in its course. Free-flowing water is denoted by hydraulic engineers as an M₂ or M₃ curve.

high-water marks Evidence of the stage reached by a flow. High-water marks generally consist of debris, scour marks, or staining of rocks along the channel banks.

hydrograph A graph of stage or discharge versus time.

Manning’s roughness coefficient (n-values) or Manning’s n A measure of the frictional resistance exerted by a channel on the flow. The n value also can reflect other energy losses such as those resulting from the transport of material and debris, unsteady flow, extreme turbulence, that are difficult or impossible to isolate and quantify.

normal depth The depth of flow for a given discharge, fixed channel shape, slope, and roughness.

particle-size The size of material on the bed of a stream, referenced to a specific diameter (either maximum, intermediate, or minimum) of the measured particle.

peak flow See runoff peak flow.

reach A length of stream that is selected to represent a uniform set of physical, chemical, and biological conditions.

river bank The sloping ground that borders a stream and confines the water in the channel. A river bank is bordered by the floodplain and channel.

runoff peak flow The largest value of the runoff flow, that occurs during a flood, as measured at a particular point in the drainage basin.

slope The change in elevation per unit change in the channel’s length.

stage The height of a water surface above gage datum; same as gage height.

stage-discharge relation The relation between the water-surface elevation and discharge.

steady flow Discharge and depth of flow does not change with time or during a selected period of time.

streamflow, discharge, or flow A general term for water flowing through a channel.

subcritical flow If the flow is subcritical, flow depth is greater than the flow depth in critical flow. Inertial forces are less than the gravitational forces.

thalweg A line connecting the lowest points along the length of a riverbed. It can be quite sinuous and wander within the channel.

transient flow Discharge and depth of flow changes with time.

water-surface curves or profile Longitudinal plots of the water-surface elevation as a function of distance downstream through a channel reach.

water-surface elevation Height of a water surface above a datum; same as adding stage and NAVD 88 datum for example.

velocity-stage-discharge relation Relation between velocity, water-surface elevation, and discharge.

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