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Deltas – Landforms Built Where Stream Enters Body of Water (Base Level)

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Deltas – Landforms Built Where Stream Enters Body of Water (Base Level) Today’s plan: - Review concepts from Friday (Stream discharge, rating curve, hydrograph) - Stream table - Stream table exercise (due Thursday 5/9) Announcements: - Xhour on Thursday (5/9) - Exam 2 is on Friday (5/10) - TA-led review session Thursday (5/9) 4-7 pm in Fairchild 414 - Undergrad assistants’ office hours Thursday (5/9) 7-9 pm in Fairchild 414 Stream Discharge The amount water flowing in a channel - Volume of water passing through a plane over a certain time - Typically measured in ft3/s or m3/s Q = Ac x V Q = Discharge (m3/s) 2 Ac = Cross-sectional area (m ) V = Average stream velocity (m/s) Measuring Stream Stage and Discharge Fig. 17.13 W. W. Norton Rating Curve http://upload.wikimedia.org/wikipedia/commons/b/b4/Rating_curve.PNG HYDROGRAPH Q = Ac x V 2.9 “ rain Stream Table • Small-scale model of stream processes • “Models” are not perfect – What in this model is similar to reality? – What is different? Stream Power (Ω = ρgQwS) Ω = Stream Power (Watts) ρ = Density of water g = Acceleration due to gravity Q = Discharge S = Slope of the stream “reach” “reach”= an interval along the length of a stream Stream formation, erosion • Erosional capacity of a stream, channel formation, downcutting, headward erosion Channel Formation 1. Stream flow begins as water moving downslope - sheetwash 2. Sheetwash erosion creates tiny rill channels 3. Rills coalesce, downcut and erode headward Channel Development • Headward erosion – channel lengthens at its origin • Tributaries form, flow into Trunk • Develop drainage network Stream Longitudinal Profile • Stream gradient, base level, delta formation, river terraces Stream Gradient – slope of a stream channel Base Level – the lowest elevation a stream channel floor can reach Can be “local” - lake or dam “Ultimate” base level is sea level Deltas – landforms built where stream enters body of water (base level) The stream stops flowing and sediment is deposited Mississippi Delta Ganges Delta Nile Delta Base-level changes: Rises – stream will deposit sediment (alluvium) Lowers – stream will downcut (erode in to substrate/alluvium) Form river terraces Stream Channel Shape • Meanders, cut banks, point bars, oxbow lakes Straight streams are usually not natural http://www aud ucla edu/programs/m arch ii degree 1/studios/2013 2014/mayne/?tag=la-river Curved (Natural) Streams Velocity is not uniform in all areas of a channel Thalweg – the deepest part of a channel Commonly velocity is fastest in the thalweg Natural Streams form Meanders (snake-like curves in channel) Form in areas where the stream gradient is low (streams are not doing headward erosion or downcutting, so streams erode side- to-side) Curved (Natural) Streams In curved channels, maximum velocity occurs at the outside of the curve • The outside curve is preferentially eroded and deepened (forming a cut bank) • Sediments are deposited on the inside of the curve where velocity is low (forming a point bar) Erosion and deposition in curved channels Cut Bank Point Bar Thalweg Oxbow Lakes Form where cut banks erode toward each other until the stream breaches the region between them This isolates the meander and can form an oxbow lake Oxbow lakes.
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