Owing to subjectivity in how a channel hierarchy is

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Unformatted text preview: o subjectivity in how a channel hierarchy is interpreted (where, for example, a 1st-order headwater channel begins), and confusion caused by a variety of stream-ordering systems, the use of stream order, which was extensive in the 1950s and 1960s, is now limited. Stream power, the ability of flowing water to accomplish work (sediment transport, erosion), is the product of discharge and water-surface slope; stream power, per unit length of channel, is typically expressed in watts per meter (W m-1). 43 Subcritical flow, or tranquil flow, in open channels occurs when the water velocity is lower than that of critical flow and the Froude number is less than 1.0; in subcritical flow, ripples caused by a water-surface disturbance move both upstream and downstream. Sublimation, as applied to hydrology, is the direct conversion of ice or snow to a gaseous or vapor state. Subsidence, as an earth-surface process, is the gradual lowering or sinking of a land area, with little or no horizontal component, owing to steady compaction of subsurface rock or sediment. Natural subsidence can result from a variety of causes including solution of carbonate or evaporate rocks, erosion and weathering processes of soil and subsoil materials, and processes related to tectonism, volcanic activity, and freeze-thaw cycles. Induced subsidence may occur beneath areas underlain by soft, easily sheared rocks, especially poorly consolidated alluvium of the Basin and Range Province, where water (or gas or oil) have been extracted, thereby reducing the buoyancy effect provided by the subsurface fluid; similarly, if a large volume of water is introduced to porous, unsaturated alluvial beds, the added weight and reduction of shear strength may cause steady compaction of the underlying alluvial beds. Supercritical flow, or rapid flow, in open channels occurs when the water velocity is greater than that of critical flow and the Froude number is more than 1.0; in supercritical flow, ripples caused by a water-surface disturbance all move downstream. Supply-limited is a term to describe the short-term ability of a stream to transport a specified type of fluvial sediment or a specified range of particle sizes of sediment; when a supply-limited condition occurs, the sediment load, or flux, is limited by the amount of sediment that is available for entrainment and transport, not by the ability (or competence) of the stream to move the sediment. Surface water is water evident above the land surface either flowing in a channel or collected in a lake, pond, or reservoir. Suspended sediment, or suspended load is sediment moved in suspension in water and is maintained in suspension by the upward component of turbulent currents or by colloidal suspension. The regulatory acronym for suspended load is Total Suspended Solids (TSS). T Teleconnection refers to cause-and-effect empirical relations among weather patterns and characteristics in various parts of the world resulting from variation in sea-surface temperature; for example, there is likely teleconnection between summer precipitation in southwestern North America and El Niňo events of the Pacific Ocean. Terrace is a valley-contained surface that typically is expressed as a long, narrow, nearly level or gently inclined landform bounded along the lower edge by a steeper descending slope and along the higher edge by a steeper ascending slope; a terrace is always topographically higher than the flood plain, and is inundated by floods of greater magnitude than the mean annual flood. An alluvial terrace is an aggradational feature, is composed of unconsolidated to poorly consolidated alluvium and its weathering products, and generally reflects an abandoned floodplain surface; a strath (from the Gaelic word for wide river valley) terrace is an erosional 44 feature formed by stream incision into a bedrock surface, and may have little or no relation to a former flood plain. Thalweg is the line within a stream channel connecting the lowest points at all sites of the channel. The First Law of Hydrology: Everything leaks. The Second Law of Hydrology: There are never enough data. Till is that portion of drift deposited directly by glacial ice; till generally lacks stratification, is poorly sorted or unsorted, and is formed of all sediment sizes, the largest of which may be poorly rounded. Tinaja refers to an ephemeral water pocket or scour pool in the American Southwest that develops below a waterfall, generally in bedrock, or by minor spring flow or seepage inadequate to sustain streamflow. Total load refers to the total amount of any physical or chemical constituent or contaminant that is transported by a moving fluid, generally water as streamflow. The term most typically is used to characterize sediment discharge in streams, for which it is the combined fluxes of suspended sediment and bed load, expressed often in (metric) tons per day. Total Maximum Daily Load (TMDL), a term from Section 303(d) of the Clean Water Act, is the maximum amount of a pollutant, including sediment, that a water body can receive and still meet water-quality standards for its designated use. Total sed...
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