Drainage Basins and Hydrology
I.	Drainage basins and drainage networks.
	A.	A drainage basin is the terrain surrounding a stream valley that contributes the water and sediments carried by the stream. Sometimes called a watershed. Drainage basins are surrounded by drainage divides, or topographic high points in the landscape that funnel water downward into a stream valley.
			In North America, the continental divide is a major drainage divide, separating basins that drain westward into the Pacific Ocean from those that drain eastward into the Atlantic Ocean (or Gulf of Mexico). It follows the high peaks of the Rockies from Mexico to Canada
	B.	Drainage networks exhibit a variety of patterns of organization (see overhead):
		1.	Dendritic pattern: arrangement of tributaries and main rivers like the branches on a tree. Typical of mature drainage basins in regions of relatively shallow slope and uniformly erodable material. Think of this as a normal, or standard pattern.
		2.	Factors that can alter drainage pattern:
			a.	Time: If the drainage basin is young (like after a glacier has plowed over and obliterated the former topography), a deranged drainage pattern will appear. The topography is young, not yet eroded and developed into an efficient drainage network. Characterized by lots of scattered lakes, wetlands, bogs, and wandering stream channels.
			b.	Steep slope: If slopes are steep, gravity will sculpt drainage networks that run linearly downhill. This can produce parallel drainage (an steep inclined plane, like western Great Plains), or radial drainage (a steep cone, like a mountain).
			c.	Differential rock resistance. If alternating weak and resistant rocks are exposed at the surface, drainage networks will preferentially erode into weaker materials. This can produce rectangular drainage (following the geometry of jointing patterns), or trellis drainage (in regions of folded strata).
II.	Hydrology is the study of the movement of water throughout the physical environment.
	A.	We have already introduced the hydrologic cycle and understand water flows such as precipitation, evapotranspiration, and runoff.
	B.	In this section, we want to focus on runoff, especially the ways in which runoff can act to erode, transport, and deposit sediments across the earth's surface.
	C.	Runoff from continents occurs as both surface flow and groundwater flow. Where the water table intersects the earth's surface, we find features like springs, streams, rivers, and lakes.
	D.	Surface runoff can be further subdivided into two distinct types:
		1.	Overland flow (or sheet flow), which moves down slopes and is not confined to channels. Overland flow erodes sediments from slopes and delivers them to stream valleys.
		2.	Stream flow, which is normally channelized (except during floods). Streams erode their channel (and sometimes the surrounding floodplain), eventually carrying sediments downstream and out to the ocean by the force of gravity.
III.	Hydrology and Drainage Basin Characteristics
	A.	Quick Review -- What happens to precipitation?
		1.	Some precipitation returns to the atmosphere as evaporation. This dominates in arid regions.
		2.	Some precipitation is temporarily stored on the soil surface (as puddles) or on the vegetation (intercepted water). Intercepted rainwater gradually moves to soil by stemflow and drip.
		3.	Some precipitation infiltrates the soil surface and becomes soil water or groundwater.
		4.	Some precipitation flows overland as sheetwash. Unconfined to channels.
	B.	Delivery pathways of water to streams
			The response of streamflow to precipitation events is dramatically influenced by the character of the surrounding drainage basin, especially as it effects the tendency for water to infiltrate into the soil or directly run off as unchannelized overland flow.
			a.	Overland flow -- unchannelized surface flow that moves rapidly downslope and into stream channels, fast delivery mechanism. As sheetwash moves over land, it erodes and transports sediments down slope.
			b.	Through flow -- infiltrated water that moves slowly through soil and groundwater to the channel, slow delivery mechanism
	C.	Factors that influence Infiltration vs. Overland Flow in drainage basins:
		1.	Substrate characteristics
			a.	texture--coarser soils (sand) have larger, better connected pore spaces. They permit rapid infiltration (3-12 mm/hr for sand vs. 0-4 mm/hr for clay).
			b.	depth of permeable substrate--deeper soils have more soil storage capacity, allow more rapid infiltration.
		2.	Vegetation cover
			a.	Dense vegetation cover allows greater infiltration rates:
				1.	more humus in soil means greater pore space
				2.	interception slows down water delivery to surface, permitting more time for infiltration
				3.	reduced exposure of soil surface to rainfall splash. (Rain splash compacts the surface and clogs up surface pores, reducing infiltration.)
		3.	Prior moisture conditions
			a.	dry soils -- pore spaces are empty, can admit water rapidly
			b.	saturated soils -- pore spaces full of water, cannot hold anymore
			c.	because of impeded soil drainage, upper layers of soil may be saturated, even though lower layers are not (a concern with clays)
		4.	Precipitation intensity
			a.	High intensity rainfall (lots of rain in a short time period) leads to low infiltration rates, since water cannot soak in as fast as it falls.
			b.	Areas with frequent thunderstorms have high overland flow and low infiltration rates.
			c.	Areas with frequent cyclonic precipitation (gentler rains) have low overland flow and higher infiltration rates.
		5.	Land use
			a.	activities that compact the soil surface (grazing, paving) reduce infiltration and increase overland flow
			b.	removal of vegetation cover (logging) reduces infiltration and increases overland flow
				1.	native grassland 57 mm/hr
				2.	grazed pasture 13 mm/hr
				3.	bare ground 6 mm/hr
IV.	Stream hydrographs
	A.	Discharge (Q) is the volume of water that moves through a stream channel in a given time interval. (units are cubic feet or cubic meters per second).
		1.	It is more convenient to think of discharge as the velocity of a stream as it moves through a stream cross-section (velocity * area = volume * time).
		2.	Since Q = v * A, discharge can be increased by
			a.	increasing the velocity of streamflow, or
			b.	increasing the cross-sectional area of the channel.
			c.	both of these things happen in a flood. The stream flows faster, and the water level rises (increasing A).
	B.	Hydrographs plot discharge levels over a period of time. These illustrate how streamflow patterns change over time, generally in response to precipitation events (or snow melt in colder climates).
		1.	annual hydrographs plot discharge over a year
		2.	this curve can be decomposed into two components
			a.	storm flow -- flood peaks associated with precipitation events. These produce short-term spikes in the hydrograph. These represent rapid rises in streamflow as overland flow is delivered to the channel following a storm.
			b.	base flow -- the more consistent, predictable level of flow in a stream over long time periods. Base flow is sustained by groundwater delivery to the channel. It may rise or fall seasonally, depending on periods of general water surplus or deficit.
				1.	after prolonged drought, stream flow may drop substantially. (In arid regions, streams are often ephemeral, flowing only during rainy periods. At other times, the stream channel is empty!)
				2.	stream flow may reach predictable seasonal peaks, such as in late winter or spring (when soil recharge is complete and most excess water is shunted to groundwater as surplus).
				3.	Again, this is accentuated in colder climates as snowmelt feeds delayed delivery of winter precipitation to the channel.
		3.	short-term hydrographs plot discharge in association with a single storm event.
			a.	typically, the rising limb of the hydrograph is rapid, whereas the falling limb is slower.
			b.	there is naturally a time lag between the precipitation event and the peak discharge (flood peak). This is because it takes some time for overland flow to be delivered to the channel.
V.	Factors that effect stream hydrograph behavior in different drainage basins
	A.	clay soils, thin soil volume ---> low infiltration ---> lots of overland flow
		1.	this results in rapid water delivery to channel, flashy flood peaks
		2.	resulting hydrograph behavior
			a.	low percentage of water in base flow
			b.	highly variability in discharge between storm events
			c.	flood peaks quickly after a storm
			d.	peak flood stage is magnified by rapid delivery of much overland flow to channel (water levels crest high and fast)
	B.	coarse-textured soils, large soil volume ---> rapid infiltration ---> majority of precipitation enters soil/groundwater and is delivered to channel as throughflow.
		1.	this results in slower, more consistent delivery of water to the channel
		2.	resulting hydrograph behavior
			a.	higher percentage of water in base flow
			b.	less variability in discharge between storm events
			c.	flood peak after a storm is delayed
			d.	discharge at peak flood stage is reduced (water doesn't rise as high or as fast)
	C.	Other factors that affect the proportion of overland flow to runoff will produce the same types of effects on short-term hydrographs:
		1.	Land-uses that increase compaction or make surface impervious to infiltration will increase flashiness of discharge
			a.	Grazing
			b.	Logging
			c.	Paving
		2.	Removal of vegetation cover will increase flashiness of discharge.
		3.	Heavy rain or previously saturated soils will divert more water to overland flow, increasing peak discharge and speeding water delivery to channel.