River Systems and Landforms

Important Fluvial Terms Drainage basin/catchment/watershed: defined by the ridges that control the direction of precipitation drainage. Every stream has a drainage basin. A drainage basin collects water, which is delivered to a larger basin, creating larger streams Continental Divide: The line separating subcontinental-scale drainage basins Water and sediment usually terminate in oceans Internal drainage basins are areas in which water does not terminate in an ocean. (leaves via evaporation or subsurface gravitational drainage http://atlas.gc.ca/site/english/maps/freshwater/distribution/drainage

Drainage Basins Red: selected drainage basins for first order streams (collection of red areas should fill the yellow area but some streams not represented) Yellow: larger drainage basins for river

Drainage density is determined by dividing the total length of all streams by the area of the basin For a given surface, a higher drainage density is generally found in a humid area than in a dry area Drainage Pattern is the arrangement of channels in an area. Drainage pattern is determined by: Slope of the catchment Rock resistance to weathering Climate Underlying bedrock Subsurface hydrology

Drainage Patterns 1. Dendritic Tree-like pattern [Fig. 11-6 (a)] Efficient –branch length minimized 2. Rectangular A faulted and jointed landscape directs streams along right angle turns [Fig. 11-6 (e)] 3. Trellis Forms where resistance of bedrock varies or along a folded landscape Folds create parallel large streams, capturing runoff from smaller streams and joining into larger rivers at right angles [Fig. 11-6 (b)]

4. Radial Drainage Streams flow from central peak or dome 5. Annular Drainage Occurs in dome structures with concentric patterns of rock strata 6. Parallel drainage [Fig. 11-6 (e)] Steep slopes - similar to dendritic, but steep slopes cause branches to appear almost parallel to one another 7. Deranged Drainage In areas with disrupted surface patterns there is often no clear drainage geometry (common in glaciated areas)

Flow velocity: A measure of how fast a stream moves downstream (v in m/s). It depends on the discharge, slope, size and shape of the channel. Discharge: The amount of water flowing through a cross section of a stream (Q in m3/s). Fluctuates seasonally and diurnally (eg. melting greatest in spring/summer and during the day) Q=wdv Capacity: The amount of sediment that can be carried by a stream (m3/s or kg/s). Capacity increases with discharge. Competence: The maximum particle size that can be carried by the stream (related to flow velocity)

Sedimentary load is the total amount of sediment carried by a stream. Sedimentary load is carried by bedload, suspended load and washload. Bedload: Coarse particles (eg. sand) which have high settling velocity. Sediments are transported near the streambed, kept loose by turbulence and particle interaction. Suspended load: Particles are in the water column, sorted by weight (larger particles near the bottom). The higher the discharge, the higher the suspended load. Washload: Fine particles with low settling velocity, which travel at the same speed as the flow. Almost independent of discharge.

Solution load: The total amount of dissolved ions transported by a stream. Determined by nature of source material and the physical and chemical characteristics of the stream.

Sediment bulking: Sediment is added to a stream in excess of its capacity. In normal, turbulent flow, the critical point of the dynamic equilibrium between erosion and deposition is exceeded with sediment addition, thus, deposition (aggradation) occurs. Aggradation may lead to the development of a braided stream.

Braided stream, S. Alaska Photo: M. Miller (U. Oregon)

Flow Regimes: Reynolds Number (R): Understanding of different flow regimes helps us to understand resulting sediment bedforms and structures. Reynolds Number (R): R = inertial forces_____ = lv internal frictional forces  where  = fluid density l = flow depth v = overall mean velocity  = viscosity of the fluid Laminar flow: Quasi-laminar flow occurs in slow moving (R<500) streams with smooth bottoms. Frictional forces exceed inertial forces (500<R<2000) Turbulent flow: Higher flow velocity and bed roughness, with (R>2000) eddies. Friction with bed reduces v. Velocity constant with depth except basal laminar layer.

Flow Regimes A. Laminar flow B. Quasi-laminar flow C. Turbulent D. Obstruction in turbulent Relative velocity profiles

2. Froude Number (F): F = inertial force = v gravitational force (gl)-2 The ratio between the average velocity of the entire liquid mass and the rate of downflow wave migration (celerity). F<1 Inertial forces are less than gravitational forces. Therefore, celerity of surface waves exceeds stream velocity (ie., waveforms faster than river, and break downstream) F=1 Critical flow (no whitewater) F>1 Main body of water moves faster than surface waves. Surface waves break upstream.

Turbulent flow A. Subcritical (waves break downstream) B. Critical (standing waves at water surface) C. Supercritical (waves break upstream) CELERITY FLOW VELOCITY

Quasi-laminar flow: Sand ripples with ripple cross-lamination form, unless the sand is very coarse. Turbulent, subcritical flow: Sand dunes showing cross-beds in section form Turbulent, critical flow: Horizontal beds with plane laminations and minute strings of particles aligned parallel to flow direction at surface Turbulent, supercritical flow: Poorly-developed dunes or other massive beds form with steeper side facing upstream

How can we tell which type of flow was occurring by looking at a cross section ? In the following slides, we will look at the sediment facies resulting from (a) meandering streams, (b) decelerating streams and (c) shallow streams with highly variable water and sediment discharge.

Meandering streams Streams take on a sinuous form where slope is shallow. Water flows faster around the bends. The varying current causes distinct erosional and depositional surfaces 1. Outer bend Erosional, supercritical flows - undercut bank 2. Middle-outer Deposition of coarse-grained or parallel, laminated sand (critical flow) 3. Middle-inner Depositional, subcritical flow Cross-bedded sand dunes 4. Inner bend Fine-grained, cross-laminated sand ripples (point bars located here) As the meander bend migrates, the lateral sedimentary facies stack vertically. In other words, as the bend increases, coarse-grained material becomes covered with progressively finer-grained sediment.

Meandering Stream

As this stream meanders, the bend seen in cross-section A extends further to the left. As this occurs, there is a transition from erosional, supercritical flows to critical and subcritical flow, with the associated depositional features superimposed as in cross-section B. *Note: Plane beds and antidunes are only formed when flows are charged to capacity.

Bar Formation A. In high-competence flows in shallow streams with variable water and sediment discharge, coarse grained sediment forms upstream from finer-grained sediment (sheet bar). Such bars are parallel to current with no flow separation bubble (B) C. Finer-grained sediment tends to form transverse bars of various shapes and do contain inclined depositional surfaces downstream (D)

Bar formation

Cut and fills Hydraulic conditions change during floods, eroding bars to form Scours Coarser material is deposited during the high flow, with progressively smaller particles deposited thereafter New bars form and finer sediment is again found above the gravel, resulting in cut and fills

Oxbow lake A lake that was formerly a channel of a meandering stream Formed when a meandering stream erodes back upon itself, straightening the main channel. The old river channel is still filled with water until sedimentation fills it again

Oxbow Lake, Milk River, MT (just south of Canadian border)

Stream gradient A stream usually has a steeper slope upstream and a gentler slope downstream, resulting in an uneven, concave shape Nickpoint A nickpoint is the location at which an abrupt change in stream gradient occurs Waterfall At a nickpoint, the water falls to softer, more easily erosive rock strata, leading to undercutting

Floodplains Flat, low-lying areas near a river that are repeatedly flooded. Rivers overflow during high flow and deposit sediment upon the floodplain. Rivers of a floodplain are generally embedded within the sediment of the floodplain itself. Levees develop along the banks of rivers as a result of flooding. When a river floods, the velocity is reduced beyond the bank, leading to sedimentation. The larger particles fall first, leading to the creation of a natural levee. The river may rise relative to the floodplain, leading to backswamp areas.

Terraces Uplift may allow a stream to cut deeper into its own floodplain (rejuvenation), leading to alluvial terraces. Such terraces look like steps above the river (Fig. 11-19) River Deltas The velocity of a river rapidly decelerates as it reaches a large body of water. This leads to deposition, of progressively smaller particles (large ones first). A characteristic triangular shape forms (hence the term delta) The river channels divide into smaller ones in all the sediment, leading to what appears to be a reversed dendritic drainage pattern (braided delta).

Nile River Delta