Definitions

Accretion: The gradual addition of new land to old by the deposition of sediment carried by the water of a stream.

Aggradation: The process of building up a surface by deposition.

Alluvial: Pertaining to or composed of alluvium, or deposited by a stream or running water.

Annual flood: The arithmetic mean of all the annual maximum discharges.

Anticline: A fold, generally convex upward, that has a core containing stratigraphically older rocks.

Aquiclude: A body of rock that will absorb water slowly but will not transmit it fast enough to supply a well or spring.

Aquifer: A water-bearing stratum of permeable rock, sand, or gravel capable of providing a consistent supply of water.

Bankfull discharge: The flow at which water just fills the channel without over-topping the banks. The bankfull stage can be significant because it represents a break-point where processes governing channel and floodplain formation occur in alluvial valleys. In stable self-formed channels, bankfull discharge may correspond closely with the effective discharge.

Base flow: Water that percolates into the ground and is conveyed to the stream slowly over long periods of time, thereby sustaining streamflow during periods without rainfall.

Channel capacity: The maximum flow that a given channel is capable of transmitting without overtopping its banks.

Channel: A landscape element consisting of two banks and a bed that is capable of conveying confined surface flows downstream in a watershed. The morphology of a channel can be formed and maintained by incision associated with hillslope erosion or by processes of erosion and deposition within alluvial valleys.

Discharge: The rate of stream flow at a given instant in terms of volume per unit time.

Dolomite: A sedimentary rock that is commonly associated with limestone and consists of more than 50% by weight of the mineral dolomite (CgMg(CO₃)₂).

Drainage basin: The land area that drains water, sediment, and dissolved materials to a common outlet. The term is synonymous with watershed and catchment.

Drainage density: Ratio of the total length of all streams within a drainage basin to the area of that basin.

Drainage network: The hierarchical pattern of channels that drains a watershed. These patterns can be described as dendritic, parallel, trellis, rectangular, radial, annular, multi-basinal, or contorted, depending on the overall pattern of connectivity and shape of the network form.

Equilibrium: Conceptual term used to describe landform behavior over cyclic time spans (See definition for graded time span). The adjective "dynamic" is used to describe the progressive long term change in the landform, such as reduction in longitudinal slope, over a cyclic time span. "Equilibrium" refers to the steady state condition in which the landform fluctuates around an average condition over a graded time span within the longer cyclic time span. Accordingly, the term describes a condition under which a landform fluctuates around an average rate of change over a long period of time. The term was modified after its original conception to "dynamic meta-stable equilibrium" to account for the influence of thresholds that result in abrupt changes to landforms during cyclic time spans.

Eddy: A circular current of water running contrary to the main current, such as a whirlpool.

Effective discharge: The discharge that transports the most sediment in a channel reach over an extended period of time, usually considered to approximate a 100 year time span, thereby conforming with the concept of a graded stream (See definition). Conceptually, infrequent discharges (i.e., 100-year recurrence interval) affecting a graded stream channel transport more sediment, but seldom occur over a 100 year period. Frequent discharges (i.e., less than the 1 year recurrence interval) occur often over a 100 year period but are incapable of transporting significant quantities of sediment. Accordingly, the effective discharge of some alluvial streams has been found to be correlated with moderate discharges (i.e., 1 to 2 year recurrence interval) in regions with temperate humid climates.

Eutrophication: A process through which excessive plant growth, typically algae, induced by excess nutrients is followed by the decomposition of vegetative material and the depletion of the water's oxygen supply.

Facies: A term used in fluvial geomorphology with respect to sedimentary facies, which are designated stratigraphic units on the stream channel bottom that exhibit characteristics significantly different than those of other parts of the stream channel bottom.

Fault: A fracture or fracture zone along which there has been displacement of the sides relative to one another parallel to the fracture.

Floodplain: The portion of the river valley adjacent to the active channel that is built of sediments deposited during the present regimen of the stream and is covered with water when the river overflows its banks at flood stages.

Fold: A bend or plication in bedding, foliation, cleavage, or other planar features in rocks.

Geomorphology: The science that treats the general configuration of the earth's surface, including the classification, description, nature, origin, and development of landforms and their functional relationships to underlying structures.

Graded stream: A stream that has a slope and dimensional characteristics adjusted to provide, with available discharge, just the velocity required for the transportation of the sediment load supplied from the drainage basin over a period of years.

Graded time span: One of three time spans used to conceptually describe landform evolution. "Cyclic" time spans encompass major periods of geologic time under which a stream system undergoes a progressive change, such as a reduction in longitudinal slope. "Graded" time spans refer to a relatively short periods (i.e., approximated by 100 - 1000 year periods) on the cyclic time scale during which an equilibrium condition is approached. "Steady" time spans refer to very short periods characterized by static equilibrium during which a landform does not change.

Hydraulic jump: Term used to describe flow conditions characterized by a stationary, abrupt turbulent rise in water level in the direction of flow. See "super-critical flow" during which an equilibrium is approached

Hydrograph: A graph showing stage, flow, velocity or other characteristic of water with respect to time. A stream hydrograph commonly shows the rate of flow over time.

Hydrology: The science of the distribution and effects of water in the atmosphere and in soils and rocks.

Igneous rock: Rock that solidified from molten or partly molten material, such as volcanic magma.

Infiltration: The flow of a fluid into a solid substance through pores or small openings.

Intermittent stream: A stream that does not have a continuous flow throughout the year.

Lithology: The description of rocks on the basis of such characteristics as color, minerologic composition, and grain size.

Mass wasting: Term used to describe the downslope movement of soil and rock material under the direct influence of gravity.

Meander: One of a series of sinuous curves or loops in the course of a mature stream.

Metamorphic rock: Rock derived from pre-existing rocks by mineralogical, chemical, and/or structural changes, essentially in the solid state, in response to changes in temperature, pressure, shearing stress, and chemical environment.

Morphology: The study of structure or form.

Nutrients: The elements required to support the bodily structure and metabolism of biological organisms. These elements include nitrogen and phosphorus, which can become pollutants if present in excessive quantities or result in the generation of adverse secondary effects, such as eutrophication in slow moving or standing water.

Oxbow: A closely looping stream meander having an extreme curvature such that only a neck of land is left between two parts of the stream.

Permeability: The capacity of porous rock, soil, or sediment for transmitting a fluid.

Perennial stream: A stream that flows continuously throughout the year.

Physiographic province: A region of which all parts are similar in geologic structure and climate and which has had a unified geomorphic history. Its relief structures are different from those of adjacent regions.

Reach: An uninterrupted length of stream channel with similar physical characteristics, including discharge conveyance capacity, cross section geometry, and slope.

Roughness: Features that create resistance to the downstream movement of water in a channel. The features may include sediment particles, sediment deposits, bank irregularities, the type, amount, and distribution of living and dead vegetation, and other obstructions to flow. The term is modified to "relative roughness" when the scale of the roughness elements to the water depth is considered.

Relief: The physical configuration of a part of the earth's surface with reference to variations of height and slope or to irregularities in the earth's surface.

Riparian: Pertaining to or situated on the bank of a body of water, usually a river.

Runoff: The part of precipitation appearing on the land surface or in streams.

Sediment: Solid, fragmented material that is transported and deposited by wind, water or ice, chemically precipitated from solution, or secreted by an organism, that forms in layers or a loose unconsolidated form.

Sedimentary rock: A layered rock resulting from the consolidation of sediment.

Self-formed channel: A stream channel formed by processes of erosion and deposition of sediment over graded time scales of several decades or a century. Channels that are not self-formed include incised channels created by progressive erosion or hillslope processes, steep bedrock channels that are hydraulically incapable of creating an adjacent floodplain or bar features, and artificial (engineered) canals created with a single uniform channel cross section to convey all flows.

Shear stress: Shear stress is the force per unit area acting parallel to a surface. In the case of stream flow, shear stress [τ (M/LT² or N/m²)] is defined as a product of the density of water [ρ (M/L³)], the acceleration of gravity [g (L/T²)], the hydraulic radius associated with the flow [R (L)], and the average water surface slope [S (L/L)], where the dimensions include: M = mass, L = length, T = time). That is, τ = ρ g R S.

Sinuosity: The amount of curvature in a channel defined as the ratio of the active channel length to the valley length.

Stage: The height of the water surface above an established datum plane.

Stream order: A classification of the relative position of streams in channel network, assigning each link an integer order number determined by the pattern of confluences in the tributary network.

Stream power: Stream power is the amount of work performed by the stream flow per unit time, which can be expressed relative to a unit of stream bed area or length of channel. Power is traditionally expressed in watts, which ultimately can be expressed as ML²/T³ , where the dimensions include: M = mass, L = length, T = time.

a) Stream power per unit area [ω _a (M /T³ -or- watts/L²)] can be defined as a product of the shear stress [τ (M/LT²)] and average velocity [V (L/T)]. That is, ω _a = τ V.

b) Stream power per unit length of channel [ω _l (ML / T³ -or- watts/m)] can be defined as a product of the water density[ρ (M/L³)], acceleration of gravity [g (L/T²)], discharge [Q (L³/T)], and water surface slope [S(L/L)]. That is, ω _l = ρ g Q S.

Structure: The attitude and relative positions of the rock masses of an area, the sum total of structural features resulting from such processes as faulting, folding, and igneous intrusions.

Super-critical flow: Flow condition that occurs when the velocity of the flow exceeds the velocity of a gravity wave propagated in the same medium. Flow conditions can be characterized using a dimensionless ratio of intertial to gravitational forces called a Froude number (Fr)as: "super-critical" when Fr > 1, "sub-critical" when the Fr < 1, and "critical" when Fr = 1. The Froude number (Fr) is defined as the ratio of the average flow velocity [V(L/T)] to the square root of the product of acceleration of gravity [g (L/T²)] and the flow depth [d (L)], where the dimensions include: M = mass, L = length, T = time. That is, Fr = V / (g d)^½.Standing waves that can be observed on the water surface coincide with locations where abrupt transitions from super-critical to sub-critical flow conditions occur.

Terrace: A relatively level bench or step-like surface breaking the continuity of a slope.

Transpiration: The process by which water absorbed by plants is evaporated into the atmosphere from the plant surface.

Watershed: The land area that drains water, sediment, and dissolved materials to a common outlet. The term is synonymous with drainage basin and catchment.

Wetland: Term used to describe areas that are inundated or saturated by surface or groundwater at a frequency and duration sufficient to support, and under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions, including swamps, marshes, bogs, and other similar areas.

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This page was updated on 4/12/2006