Erodible channel boundaries (alluvial banks and bed)
Transport Capacity Sediment Supply
Storage can be quite high
Importance of alluvial riversRivers:Provide water and nutrients for agricultureProvide habitat to diverse flora and faunaProvide routes for commerceProvide recreationProvide electricity
Bankfull DischargeTypically bankfull discharge equates to a roughly 2-year recurrence interval flow.
Three basic map-pattern forms of streams:
STRAIGHT CHANNELSStraight channels are rare.Straight channels form where streams are confined by topography or follow geologic structures.Generally mountains streams.
Streams generally erode on outer (cut) banks where velocity is greatest, and deposit on the inner sides of bends where velocity is slower.Meanders tend to grow as the flow erodes the banks, favoring development of meandering channels.
Loops or meanders form as stream erodes its banks.
Erosion takes place on the cut bank, which is theoutside loop of the meander.
Deposition takes place on the point bar, which ison the inside loop of the meander.
Change their channel course gradually
Create floodplains wider than the channelVery Fertile soilSubjected to seasonal floodingMeandering channels
Point BarsCut BanksMeandering channels
Meandering streams often characterized by large loopy bends across their floodplains.
Meanders occur most commonly in channels that lie in fine-grained stream sediments and have gentle gradients.
Growing meanders can intersect each other and cut off a meander loop, forming an oxbow lake.
Old channels abandoned as a river meanders across its floodplain form oxbows.Oxbow lake
Sacramento River, CAOwens River, CAMeander train = belt of meandering
Sacramento River, CAOwens River, CANote old meanders
Channel migration zone = area across which the river is prone to move.Meander train = belt of meanderingMeander beltMeander belt
Formation of Meanders
Point bar deposits
Pool - riffle sequenceRiffle to riffle = 5 - 7 channel widths
Riffles, pools, and cascadesRiffles and pools alternate in somewhat predictable patterns
Holden Crater, Mars
BRAIDED CHANNELSMany converging and diverging streams separatedby gravel bars (or sand bars).
Braided StreamsHigh sediment loadAnastamosing channelsConstantly changing courseFloodplain completely occupied by channelsMany small islands called mid-channel barsUsually coarse sand and gravel deposits.
If a stream is unable to move all the available load, it tends to deposit the coarsest sediment as a bar that locally divides the flow.
Braided channels tends to form in streams having highly variable discharge, easily erodible banks, and/or a high sediment load.
Glacial streams generally are braided because:
The discharge varies both daily and seasonally.
The glacier supplies the stream with large quantities of sediment.Braided Channels
Braided channels clog themselves with sediment, so channels always shifting
Generally in streams near mountain fronts
Variability in river systems Four dimensions:Longitudinal LateralVerticalTimeThe four dimensions of a stream system
Variation in time and spaceThe shape, size and content of a river are constantly changing, forming a close and mutual interdependence between the river and the land it traverses.
Sinuosity: Gradient and substrateBig meanderslow gradientfine substratesSmall meandershigh gradientcoarse substrates
Longitudinal Profile of Mountain Rivers
Channel typeBedrockColluvialAlluvial A. CascadeB. Step-poolC. Plane-bedD. Pool & riffleE. Dune ripple
Colluvial ChannelsSmall headwater channels at the tips of the channel network where sediment transport is dominated by landslide processes.
Cascade ChannelsThe steepest of mountain channels, characterized by tumbling flow around individual boulders; disorganized streambed structure.
Step-Pool ChannelsChannels displaying full-width-spanning accumulations of coarse sediment that forms a sequence of steps.
Plane-Bed ChannelsChannels lacking well-defined bedforms and instead displaying long reaches lacking pools.
Pool-Riffle ChannelsThe most common mountain river morphology; characterized by alternating sequence of pools and bars.
Longitudinal Profile of Mountain Rivers
Active stream channels change over time, eroding and depositing sediments in predictable patterns that are apparent in stream cross sections.
Scarp active, eroding side of stream channelThalweg path of deepest channel
Fig. 1.12 - Cross section of a stream channel. The thalweg is the deepest part of the channel. In Stream Corridor Restoration: Principles, Processes, and Practices (10/98).Interagency Stream Restoration Working Group (15 federal agencies)(FISRWG).Pools and riffles appear in a somewhat regular pattern. Riffles are separated by distances of approximately 5-7 channels widths in streams with a gravel dominated substrate.
Riffles form where the deposition of gravel bars occurs. This relates the occurrence of riffles to the sinuosity of the stream.
Rivers are very dynamic and show tremendous variation through time and space. The shape, size and content of a river are constantly changing, forming a close and mutual interdependence between the river and the land it traverses. Imagine following one river from a small trickle in the mountains, over raging rapids and waterfalls, through areas of peaceful, nearly motionless water, all the way to its meshing with the mighty waves of the sea. On its way, a river may carve through majestic mountain ranges, create deep gorges and canyons, gentle valleys, lush meadows, and mighty plains, all the while providing the setting for a diversity of biological communities. Over time it may change drastically from a roaring, overflowing force in the spring, to a still, icy-cold mass in the winter.All streams exhibit some sinuosity. The degree varies from stream to stream based on stream gradient, local geology, hydrology, and land use.Image: Montgomery & Buffington 1997, Figure 3Colluvial Channel: Bed sediments are a result of mass wasting; not transported by the river. Common in small headwater streamsAlluvial Channel: Channel bed sediments transported by the riverCascades: have tumbling flow due to dissipation of energy by large boulders in channel. High Gradient. Large bed materials are immobile during average flows while small grains are transported quickly.Step-Pool: have longitudinal steps made of boulders and cobbles. High Gradient. Steps are removed by large floods and reconstructed when flood waters recede.Plane-bed moderate gradient streams that lack discrete bars. Typically have armored beds indicating that transport capacity > sediment supplyPool & Riffle low gradient streams undulating beds. Bar/pool riffle sequences.Dune-ripple low gradient streams with ripples or dunes in the bed. Sediment transport at most flows.