unit 7 - river morphology

hydrology

the frequency-magnitude of flood events

hydraulics

relate dimensions and discharge to stress and power

graded stream

river systems have a fixed amount of discharge, sediment supply, but rivers can adjust slope/width/depth to match stream power needed to transport sediment

• dynamic equilibrium (or at least trending towards it)

at-a-station hydraulic geometry

measures a fixed point in the river and calculates variable discharge over time

• used for fisheries, habitats, and egg loss

• bridge construction

• rating curves for Q

• the equations used depend on the xs form (triangular, parabolic, bar-pool section, etc)

downstream hydraulic geometry

measures multiple points over the course of a river, but at one point in time

• channel design for restoration and urban areas

• model for channel response (changes in Q and bank material)

• fundamental scaling behaviour system

what are the types of river morphology

colluvial

alluvial (braided, pool/riffle, step-pool, cascade)

bedrock

colluvial streams

sediment is from mass wasting events

alluvial

sediment is from water transport

bedrock

sediment coming is is immediately transport out

• there is less sediment than transport capacity

bedrock streams

headwater streams

• fixed channel boundaries

• high transport capacity

• low storage

• input = output

cascade channels

steep channels (up to 35%)

• bed material is boulders

• random bed patterns

• mixed sediment source (fluvial and mass wasting)

step-pool

medium gradient (3-15%)

• steps are stable an drarely move

• largest boulders get lodged perpendicular and usually span the bank

• flow plunges over steps and carves out pools

• fine material moves yearly but large material only moves during large 10 year floods

intermediate alluvial channels

wider than particle size

• slop is under 5%

• defined by roughness elements and fluvial action

• log jams influence sediment transport (greate for fish habitat)

what is the influence of large wood

forces pool-riffle morphology where otherwise the bed would be plane

• most important where channel spanning jams are common

• hazards to infrastructure

• removing wood can reduce roughness, increase velocity, and cause a river to flow straighter

large alluvial channels

d » debris, wbf > tree height and slope is less than 1%

• determined by fluvial action

• character is dependant on sediment supply

• laterally active (erosion and deposition to floodplains)

• channel pattern is straight to braided

braided channel

made of multiple active threats separated by bars

• very dynamic

• high sediment load and erodible banks

anastomosing channels

complex pattern of individual channels that bifurcate and rejoin flow around stable vegetated islands

• deeper, narrower, and have lower gradients than braided channels

• promoted by cohesive banks

• predominantly clays and silts (cohesive)

• linked to excessive upstream bedload

what does increased sediment in rivers lead to

• flooding

• channel migration

• water quality issues

• reservoir filling

what controls channel patterns and behaviour

sediment input/output, confinement, climate, channel/valley gradient, connectivity, sediment transport

where does sediment input come from in mountain basins

• supplies by extreme mass movements and wasting events

• sediment input varies over time in size and volume depending on the source

where is sediment input from in lowland rivers

• supplied by upstream channels and local bank erosion

• Sediment input varies over time in grain size and volume

what is sediment connectivity

how efficiently does sediment move throughout the landscape

what are the geological controls on landscape

• faults control valley alignment

• lithology controls valley width

• lineaments control landsliding

what landforms does glacial history leave

• flow convergence creates valley steps

• over-steepened hillslopes, aretes, horns, etc.

what are these effects on postglacial history

• stochastic landsliding

• postglacial incision/deposition

what controls sediment storage patterns and yield

valley steps controls storage and fragment sediment cascade

• most sediment storage landforms appear to be active throughout Holocene

what is sediment yield mostly influenced by

exceptional events