geomorph

• Drainage basin


  An entire area drained by a stream and its tributaries. Fundamental unit for geomorphic analysis of fluvial system because they are the land surface over which water and sediment move down topographic gradients. Drainage basins are separated by elevated land, or ridges, between them that define topographic drainage divides.
• Floodplains


  Where sediment tends to be deposited in drainage basins
• Upper reaches of the basin

  Where sediment most likely originates - upland hillslope erosion
• Open drainage basin


  Mass (water, sediment, and dissolved load) is transported through and then out of the watershed to larger watersheds or the ocean
• Closed (internally drained) drainage basin

  Terminated in lowlands where water is lost only by evaporation and seepage into groundwater system
• Closed drainage basins are commonly found in arid regions where drainage networks not well developed, in areas where local tectonic regime is extensional, and in glaciated areas where glacial erosion has over deepened valley bottoms
• Slope processes such as landslides and gullying are episodic, and short-term/ long-term mass flux rates differ dramatically - this is one of the specific challenges geomorphologists face in creating a sediment budget
• Sediment rating curve


  Estimation of sediment discharge (amount of sediment moved by a stream in a given time) in a river based on fluvial data/ describe sediment discharge as a function of water discharge
• Sediment rating curves


  Created by estimating flow over time and measuring suspended load during a variety of different discharge events
• Hysteresis


  The variation of magnetisation with applied field and illustrates the ability of a material to retain its magnetisation even after an applied field is removed. Occurs during unsteady flow when water surface slope changes due to either rapidly rising or rapidly falling water levels in a channel control reach
• Factors contributing to variability in sediment rating curve data

  • Hydrological variability (precip patterns, snow melt, evapotranspiration)
  • Sediment supply (natural weathering processes)
  • Particle size distribution
• Drainage patterns


  • Dendritic - Appalachian Plateau in Pennsylvania
  • Trellis - valley & ridge province, Appalachian Mountains
  • Rectangular - N America midcontinent
  • Radial - Mount Rainier , volcanoes
• Dendritic drainage pattern


  Little structural control on topography, form in absence of structural or lithologic controls, so relatively flat-lying sediments or in homogenous crystalline rocks
• Trellis drainage pattern


  Structurally controlled drainage, form in areas underlain by tilted or folded beds of alternately weak and resistant sedimentary rocks. Preferential erosion along weak beds results in development of bedding-parallel strike valleys with short and steep dip and anti-dip streams incised in resistant strata
• Rectangular drainage pattern


  Similar to trellis – with two dominant discharge directions more equally developed. Jointing or faults govern drainage patterns by producing linear zones more susceptible to weathering and erosions, especially areas of carbonate rocks such as limestone
• Radial drainage pattern


  Flow away from central points, such as volcanoes
• Antecedent drainage


  As part of a river slope & surrounding area gets uplifted and the river sticks to its original slope, cutting through uplifted portion like a saw forming deep gorges. Rivers are formed even before underlying rock topography.
• Superimposed drainage


  A river flowing over softer rock stratum which reaches the harder basal rocks but continues to follow initial slope. The stream has enough erosive power that cuts its way through any kind of bedrock, maintaining its former drainage pattern. The rivers are formed over already present underlying rock structures.
• First-order stream


  A channel that regularly carries flow. Describes channel's position and ranks in drainage network.
• Issues with defining first-order channels: stream ordering is usually done from maps and not on extensive fieldwork
• Strahler stream ordering


  When two first order channels join, the channel downstream is designated as a second-order channel. When two second-order channels join, the result is a third-order channel.
• Shreve stream ordering


  Stream magnitude as the total number of first order streams contributing to reach in question. Both stream order and magnitude are related positively to discharge, stream length, channel width, depth, and cross-sectional area, and sinuosity.
• Strahler stream ordering is more commonly used than Shreve
• In humid temperate river networks

  Channel width, depth, and cross-sectional area increase downstream as a function of drainage basin area and increasing discharge
• In arid-region river networks, relationships between downstream changes in channel characteristics are often more complex because losing streams are common, so channels do not convey increasing discharges downstream
• Basin area


  Positively related to many different hydraulic geometry variables including channel width, depth, velocity, and cross-sectional area
• Stream velocity is generally greater in a large lowland river compared to a mountain cascade, due to the higher friction and energy dissipation in shallow mountain streams
• Sediment residence time in drainage basins


  Varies widely. Sediment eroded off slopes can be trapped for centuries to millennia before entering the fluvial system. Once sediment enters stream channels, its residence time is controlled by grain size and valley morphology.
• In narrow steep upland valleys there is little space for sediment to be stored, while in small lower gradient valleys rapid human-induced hillslope erosion has caused massive aggradation
• Sediment transit times in humid-temperate river channels

  Grain-size dependent. Smallest grains carried in suspension generally move at pace of river flow, except those that settle out on floodplains, which will eventually be reintroduced to the channel by point bar/cutbank migration.
• Valley segment types


  • Colluvial - sediment delivered more rapidly than fluvial processes can remove
  • Bedrock - narrow, steep with little sediment storage and high transport capacity
  • Alluvial - low gradient, filled with sediment, unable to scour to bedrock
  • Estuarine - low-gradient, wide, filled with fine-grained sediment, heavily vegetated
• The slope of a river's longitudinal profile changes in the downstream direction due to changes in total basin relief, fluvial relief, and local relief
• Base levels


  Lowest point to which a river or stream can erode its channel. Can change over time due to tectonic activity, climate changes, and human impacts.
• Knickzone


  Areas where the river bed is steeper than up or downstream, a cascade or area of fast water. Can reflect faulting or the presence of strong rocks resistant to erosion.
• Knickpoint


  Discrete jumps in elevation along a river's bed, or waterfalls. Can result from base level change, faulting, resistant rocks, or lingering effects of valley glaciation.
• Downstream in a drainage basin

  Floodplain morphology changes - valley widens, gradient decreases, energy dissipation increases, interaction with groundwater changes
• Bed material grain sizes typically decrease downstream in a drainage basin, due to abrasion, weathering, and selective transport
• Strath terraces


  Erosional terraces indicative of a geomorphic regime where the river had sufficient energy to not only move its sediment load but also cut into the channel bed material
• Depositional terraces


  Indicative of a river system where sediment supply once exceeded the capacity of the river to transport sediment, leading to deposition in the valley bottom
• Reasons why rivers leave behind terraces

  • Changes in sediment supply
  • Changes in water volume and flow rate
  • Tectonic activity