Stream Channels and Floodplains
I.	Variation in hydrographs throughout drainage basin.
		Compare streams of three different orders, such as Laramie, Platte, and Missouri River.
		A.	Nested drainage basins (drainage basins of lower order streams are subsets of drainage basin for higher order streams).
		B.	The area of the drainage basins gets progressively larger: Laramie < Platte < Missouri.
		C.	Both discharge and channel cross-sectional area get progressively larger moving from lower to higher order streams.
		D.	Their hydrographs show characteristic patterns of response to water input:
			1.	Higher order stream carries more water, so it has a higher discharge.
			2.	Higher order stream has higher peak flood discharge, but the flood peak is delayed longer from rainfall event, and the flood peak lasts longer before subsiding.
			3.	Lower order stream is flashier - floods at lower peak discharge, shorter delay in discharge peak and shorter duration of flooding.
II.	Sources and types of sediment in streams
	A.	Source - regolith/sediments eroded from the surrounding hillslopes in the watershed. Slope processes (like mass wasting) and overland flow deliver sediments to the stream.
	B.	Types of sediments in streams:
		1.	Dissolved load - material carried in water as dissolved ions, such as the calcium from limestone bedrock in karst landscapes.
		2.	Suspended load - clastic sediments held up by running water, like silt and clay particles (Our easily eroded, clay-rich soils on the piedmont supply plenty of suspended load to local rivers, which is why they look orange/brown and muddy!)
		3.	Bed load - clastic sediments that are too large to remain in suspension in running water, like sand, gravel, boulders. These materials roll and bounce along the bed of the channel. Much bed load transport occurs during flooding events.
III.	Factors affecting stream capacity:
	A.	Stream capacity - the total suspended and bed load that a stream can transport.
	B.	Stream capacity varies with:
		1.	Gradient - elevation change over horizontal distance, or slope of the stream channel.
			a.	Steeper gradient --> greater capacity.
			b.	In the short term, in the absence of dramatic erosion or deposition, gradient remains constant.
		2.	Velocity - rate of flow of water (m/sec)
			a.	Higher velocity --> greater capacity. If v is doubled, capacity increases 8-16 times!
			b.	Varies with water input. When streams flood, velocities increase substantially, so the stream can carry much more material.
		3.	Channel size and shape -
			a.	Wetted perimeter is the cross-sectional edge of the stream channel, where the stream makes contact with its channel. This produces friction.
				1.	shapes that minimize perimeter-to-area ratio have higher capacity
				2.	if shape is held constant, larger channels have higher capacity.
Area of A = 4 x Area of B Wetted Perimeter of A = 2 x WP of B
A, the larger channel, has a greater Area/WP ratio, which reduces friction and increases stream capacity.
			b.	Like gradient, channel shape and size do not vary in the short-term, as long as erosion and deposition are not dramatic.
IV.	Stream load vs. stream capacity
	A.	If capacity is less than load, the stream cannot carry its load, so it deposits part of it.
	B.	If capacity exceeds load, the stream has excess energy (gravitational, potential energy) so it can erode more sediments.
	C.	Streams switch back and forth from depositional to erosional agents, depending on load vs. capacity.
		1.	Streams can erode along one stretch and deposit along another, since gradient and channel shape/size vary along the stream's course.
		2.	Streams can erode during periods of higher velocity or discharge (floods), and deposit during periods of lower velocity or discharge.
		3.	Streams can be thought of as a sediment transport system that attempts to maintain an equilibrium, a balance between load and capacity.
				Anything that alters the sediment load delivered to the channel, or that alters the stream's capacity to carry that load, will cause the stream to adjust its gradient or channel geometry (by erosion or deposition) to achieve a new equilibrium, a new balance between load and capacity.
V.	Longitudinal profiles of streams
	A.	A longitudinal profile is a plot of stream gradient from headwaters to mouth.
	B.	Headwaters are characterized by:
		1.	Steep gradient, channel carved into bedrock/regolith.
		2.	Small channels size and inefficient channel shape.
		3.	Most load is transported as bed load, rather than suspended load. Water runs clear in mountains, but stream bed is full of gravel and boulders along channel.
				Where bed load is much greater than suspended load, the channel form is often braided, with lots of small channels running down the coarse stream bed.
		4.	Capacity is maintained by steep gradient, which helps move the bed load, especially during floods.
	C.	Mouth is characterized by:
		1.	Flat, or shallow gradient, channel flows over old flood deposits in a floodplain.
		2.	Large channels size and efficient channel shape.
		3.	Most load is transported as suspended load, rather than bed load. (More dissolved load than in headwaters, too.) Rivers are opaque, muddy in appearance, channel bed primarily sandy.
				Where suspended load is much greater than bed load, the channel form is often meandering, with a single channel that wraps sinuously across a floodplain.
		4.	Capacity is maintained by large, efficient channel, which allows higher mean velocity to erode and transport much sediment in suspension.
VI.	Streams as agents of work in the landscape:
	A.	Work done by the water in the channel
		1.	Dissolves basic cations, like calcium in limestone.
		2.	Hydraulic action of water against channel edge dislodges and lifts sediments.
	B.	Work done by transported sediments
			Particles collide with channel edge, help dislodge and abrade channel.
	C.	Work done by streams includes:
		1.	Erosion of sediments, occurs where capacity exceeds load.
			a.	In headwaters
				1.	downcutting and valley deepening
				2.	cutting into head of drainage basin
				3.	eroding resistant bedrock where it is exposed along the channel (e.g. nickpoints --> rapids and waterfalls). Nickpoints gradually eroded upstream, by combination of basal sapping and mass wasting of caprock.
			b.	Lower in drainage basin
					lateral erosion and valley widening as meandering stream channel migrates back and forth across the floodplain.
		2.	Transport of sediments
			Deposition of sediments, where load exceeds capacity.
			a.	In headwaters, load often exceeds capacity where lots of sediment is delivered to channels, like from glacial meltwaters or mass wasting. Result is braided channel.
			b.	Lower in drainage basin, meandering streams combine with periodic flooding to create a variety of erosional and depositional landforms (more to follow).
VII.	Floodplains - broad, flat valley bottoms adjacent to a stream channel.
	A.	Floodplains are formed by a combination of lateral erosion and deposition of sediments during floods.
	B.	Meandering stream channels migrate with time, because velocity varies across the channel.
		1.	On outside of meander bends: increased velocity --> increased capacity --> erosion of cutbank.
		2.	On inside of meander bends: decreased velocity --> decreased capacity --> deposition of point bar.
		3.	This asymmetrical work across the channel causes meandering streams to migrate laterally across the floodplain and down valley.
				As a meander bend erodes the channel down valley, it can actually cut off another meander.
				a.	Since the gradient is locally steepened where the meander is cutoff, the stream flows through the cutoff and abandons the former channel.
				b.	These leaves behind ox-bow lakes in the floodplain (which gradually fill with sediments during floods to form meander scars).
	C.	During floods, streams rise over the channel banks and spill out onto the adjacent floodplain.
		1.	Since flood waters have high velocity in the stream channel itself, the capacity in the stream channel is increased, and the channel may be eroded during the flood (which, of course, will change channel shape and size).
		2.	The water that spills out of a flooded channel onto the adjacent floodplain drops is velocity abruptly at the channel margin (especially as flood waters recede).
			a.	This produces deposition across the flooded area, adding a layer of sediments to the youthful, fertile soils of the floodplain.
			b.	Floodplain soils are perpetually renewed by nutrient subsidies eroded and dissolved from the surrounding drainage basin.
			c.	At the edge of the channel, where velocity drops most quickly, natural levees often form. These are elevated ridges of sand deposited as stream capacity drops quickly with reduced velocity of flood waters from channel to adjacent floodplain.
VIII.	Flood events
	A.	Floods occur when discharge rises to the point that water overflows the banks of a stream channel.
	B.	On average, streams with floodplains are flooded about once a year.
	C.	Floods, of course, differ in their magnitude, depending on how much water has been added to the drainage basin by precipitation or spring snow melt.
			Floods are rated on their probability of recurrence.
			1.	Severe floods, with high peak discharges are rare. Their probability of recurrence in any given year is low.
			2.	10-year flood has a 10% chance of occurring in any given year, 1000-year flood has a 0.1% chance of occurring in a given year.
			3.	Like all statistics, these are just summaries of probabilities. You can have two 100-year floods within a few years of one another, by chance.
			4.	The Mississippi floods of 1993 were rated as 500- to 1000-yr floods.

IX.	Some concluding observations:
	A.	With time in a drainage basin:
		1.	Irregularities in a longitudinal profile are eroded down.
		2.	Lakes are filled in by deposition. (Lakes are temporary depressions in the earth's surface. Given enough time, they will all fill. But it might take so long that tectonic uplift or some other geologic or climatic event will intervene.)
		3.	Valleys become deeper and wider.
		4.	Surrounding hillsides are eroded into flatter, smoother forms.
	B.	Relationship of peak flood discharge and work accomplished over long time periods:
		1.	Rare floods --> low frequency events --> accomplish much work (redistribution of sediments) per event --> moderate amount of work performed in aggregate over long time period.
		2.	Annual floods --> high frequency event --> moderate work accomplished per event --> many events add up to lots of geomorphic work performed, in aggregate over long time period. These events are the primary shaper of fluvial landforms.
		3.	Normal flow --> accomplishes little work, even accumulated over long time periods
		4.	Result is that geomorphic work occurs in pulses associated with floods. The big floods look most dramatic (greatest short-term sediment redistribution), but annual floods do most work over the long haul.