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River Systems March 10th & 15th Notes Pkg. Consolidated complete River Systems Lecture including topics: How Streams Work (Channel precipitation/Overland & Base flow), Energy in Stream Flow, Storm Hydrographs, Erosion in Fluvial Systems (hydraulic axn/abr

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Sarah Finkelstein

Geomorphology: River Systems and Fluvial Landforms//Chp. 14 How do streams work? -Rivers drain the landscape; plumbing system for moving water falling in uplands  lowlands -The water in rivers/stream flow comes from three main sources: 1).Channel Precipitation: rainfall 2). Overland Flow: water running off over land, not contained in a channel. Snow melt = incredible potential energy as well in areas with a lot of snowfall. 3). Base Flow: comes from groundwater when it isn’t raining. Groundwater feeds/supplies creeks. >Groundwater: little droplets of water percolating through tiny pores in the ground; slow procession of water. Groundwater is replenished by Infiltration [of ppt.], which can then seep into creeks. -Streams form networks. They start as channels that merge into branches, and eventually these networks are large enough and have expanded enough to enter rivers. -River>Stream>Creek vaguely/size-wise. -Gravity (for direction) and Insolation (for moisture) are PWRing these stream processes >Gravity: there must be some slope for water to flow downstream >drainage divides: local topographic highs (e.g. could be top of a hill) on landscape that divide where water can flow; water falling directly on top of a drainage divide will fall either left or right into one of two separated stream channels. >Insolation/solar radiation: Rainfall comes onto land b/c of evaporation, PWRed by insolation. Most evaporation happens over the ocean. B/c of advection of moisture into clouds, rainfall occurs over land. -Stream discharge: is the volume of water passing through a given cross-section of stream channel in given amt. of time. Measured in m / s . -1 >Stream discharge = Q. Q=wdv = (width of section[m])(depth of water[m])(velocity of flow[m s ]) > also denoted by Q = av = (area)(velocity). Use the equation to figure out how Q changes in case it does -Q is the volume of flow in a given unit time. The flow can speed up (if water flows over rapids/steep slope) or slow down (if water flows into a pool area of channel). -Q remains constant unless you’re losing (evaporation)/gaining (incoming tributaries) water. Storm hydrograph -Lag time: tells us there’re other tributaries that are also receiving the large amount of storm water, having yet to feed all that extra water into the stream being studied. Lag time reflects the time needed for the excess water to pass through the watershed, that is, through all the other tributaries leading to a particular stream. Sometimes water passing through the watershed takes a while and other times it’s shorter, reflecting the amount of tributaries necessary for water to flow through before finally depositing in a particular stream. -Base flow: also takes a long time because it recharges groundwater, which has to percolate back deep into the ground through infiltration, and has already been set Geomorphology: River Systems and Fluvial Landforms//Chp. 14 back by lag time. Thus the base flow increase happens slowly and over time. -River floods: a flood occurs when water can’t fit in a stream channel, which has a certain depth. It’ll spill onto flat adjacent land, called the floodplain. Flatter the floodplain, wider flood DMG/flood will spread. >Flooding occurs when value of Q exceeds cross-sectional area. >bigger floods are rarer and smaller floods more common > every 50yrs. or so is how much huge floods of 500 cubic metres will occur -Stream competence: ability of stream to move particles of a specific size e.g. In streams where there are large rocks and little water/flow, indicative of greater flow in the past, capable of moving large particles. e.g. circular rocks indicate weathering by flowing water; edges of rocks have been worn DWN so more water can flow, so chemical weathering/erosion over long period of time has occurred >The higher the bank/elevation, the better water can flow through [as opposed to flatter floodplain conditions] Energy in Stream Flow - potential energy of flow; water does work when it falls and slope doesn’t have to be necessarily super-steep for this to occur. Potential energy = (mass)(gravity) (height). -Fluvial Processes = the work that rivers do: move rocks and sediments (erode/deposit). Energy dissipates when work is done. -more PE = more erosion. -Base Level: ground zero for PE >Ultimate Base Level = Sea LVL b/c all sediment will eventually circulate towards/into the ocean from rivers >damming creates artificial base level -Rising or falling sea LVL. Affects stream flow >Stream Entrenchment: when sea LVL lowers, streams have larger h term, steeper, more PE to use for erosion. Streams gain more PWR, carving deeper valleys of streams > Valley Infilling: when sea LVL rises, all rivers flowing into it get backlogged and flood; water goes upstream. Streams have smaller h term, less steep, can’t do as much work. Sediment build-up due to reduced erosion; choked stream -Streams erode and deposit material; creating erosional and depositional landforms through fluvial processes. Erosion in fluvial systems -River systems exist b/c landscape needs to be drained. -Rivers do work. Rivers carve valleys and create hydroelectric energy etc. - 3 main ways we think about flowing water eroding/carving out stream channel(s): 1).hydraulic action: drag of flowing water on banks dislodges sediment 2).abrasion: suspended material bounces along stream channel, aiding erosion by gouging away at the surfaces. Consider it “liquid sand paper” Geomorphology: River Systems and Fluvial Landforms//Chp. 14 3).corrosion (chemical weathering): much water as we know is slightly naturally acidic and can be quite corrosive. What is sediment? Sediment: unconsolidated particles, grains of rock transported/deposited by agents of erosion; eolian agents, fluvial/glacial/coastal (waves) agents >size-specific sediment: boulders (256+ mm) to clay (0.0039-0.00195mm) particles *know the sizes of sediment, as in which is bigger and which direction chart goes sizewise* -4 ways to transport sediment in stream channels: 1). In solution (dissolved load): ions in river water, excess nutrients from runoff of agricultural areas etc. 2). In suspension (suspended load): particl
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