This preview shows page 1. Sign up to view the full content.
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
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
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
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.
View Full Document
- Fall '13
- The Adventures of Tom Sawyer