Fluvial GeomorphologyEnvironmental HydrologyLecture 20

Fluvial GeomorphologyStudy of the role of running water in shaping the landscape

Key concepts in fluvial geomorphologyCharacteristics of rivers as function of landscape position (geography)How rivers accomplish workHow rivers respond to change

Geomorphic WorkRunning water exerts force Landscape offers resistance

Geomorphic WorkLanes Balance, Ward & Trimble, Fig 9.20Q S a Qs dWhere: Q = dischargeQs = sediment dischargeS = sloped = median sediment sizeQ*S = stream power

When does geomorphic work occur?after Wolman, M. G. and J. P. Miller, 1960. Magnitude and Frequency of Forces in Geomorphic Processes. Journal of Geology, 68(1): 54-74.

When does geomorphic work occur?see Ward & Trimble, Fig 6.14

Effective width, depth

Geomorphic work expressed in channel characteristicsChannel dimensions (cross-section)

Channel profile (longitudinal view)

Channel pattern (plan view)Winooski Falls, Photo by Jim Westphalen

Channel dimensionsGeometry or cross-sectional shape of the channel Width, depth Bankfull width, depth (i.e. effective) Floodplain Terrace (abandoned floodplain)see also Ward & Trimble, Fig 6.13

Helical flow creates variations in shear stress across the cross section

Channel dimensionssee also Ward & Trimble, Fig 6.13

What happens if force-resistance balance is disturbed?

Channel evolutionGraphic from A. Ward after Schumm

Channel ProfileElevationDistance downstream

Longitudinal relationship between elevation and distance downstream (slope)

Channel ProfileWard & Trimble, Fig 6.8

Channel ProfileWard & Trimble, Fig 6.27(from Montgomery & Buffington, 1997. GSA Bulletin, 109, 596-611.)

Channel PatternMeandering StreamBraided StreamPlan (aerial) view of channel geometryWard & Trimble, Fig 6.10

Channel PatternSinuosity (S) = Lc / Lvwhere Lc = channel lengthLv = valley length

Helical flow induces meander formationCut banks

Meander GeometryWard & Trimble, Fig 6.11

How do these concepts scale?Graphic from A. Ward

How do these concepts scale?Ward & Trimble, Fig 6.15Hydraulic Geometry of stream channels