Section 2*

• What is a drainage area, and how can it be defined using a topographic map?
Drainage area is an area that contributes to a single stream down hill, connected by ridgetops, water flows either into one or the other drainage at the ridgetop you can find drainage areas using topographic maps by tracing the highest elevations surrounding a stream and decide that all of the water precipitating in this area flow downhill, down the topolines, to a single drainage area that CONTRIBUTES FLOW TO A SPECIFIC STREAM. Land area from which water flows to a common point. Definition of a watershed is a drainage area that ‘contributes..’
• How does the USGS classify watersheds?

USGS has hydrologic Unit Code HUC’s,
0-21 regions (each big region divided into subregion)
Subregion has 4 digit code ? Accounting units 6 digits? cataloging units 8 digits (poudre watershed)
Basins >1000km^2 >Waterheds>10 km^2> Catchment

• Explain the Strahler method of stream ordering.
1. head water channels with no upstream tributaries, 1 order
2. when two 1st order streams meet, becomes a 2nd order stream.
3. if 1st order and 2nd order stream join, confluence takes the larger orders name. 2nd order
4. if two 2nd order streams meet, then order becomes 3rd order.
• Define drainage density, and describe what type of climate might be associated with high/low drainage density.
Density= total length of streams/watershed area
High drainage density-over this surface lots of drainage channels, wet so thousands of channels small and big

Low drainage density- only 2 rivers on whole map, driest in the world

Drainage Density: Ratio of the total length of all channels in a drainage basin (divided by) to the basin area.
?High drainage density: Will have a more, ‘flashy,’ hydrograph with a steep falling limb. It can indicate a greater flood risk. Limited vegetation cover, large channel frequency.
?Low drainage density: Less dramatic graph, gentle slopes, lower channel frequency, much vegetation cover.

• Calculate a stream gradient using information from a topographic map.
Gradient=%=Drop/length=965ft/.7 miles x mile/5280ft = .26×100=26% VERY STEEP
Drop- 11,765 feet-10800ft=965 ft
Length, measured on map between elevations- length .7 miles.
• What other features are important descriptors of watershed characteristics?
from summary slide

-scale of consideration, drainage area, steam order
-physical characteristics, shape of stream network, stream length, drainage density, stream gradient
-land characteristics, soil type, underlying geology, land cover/ land use
-climate description

Other features important to descriptors of watershed characteristics: Soils – what type, how much water infiltrates into soil, how much water is stored in the soil, plant water availability, erodibility. Geology – bedrock and alluvium groundwater, flow pathways.
Vegetation and land cover – affects transpiration and how water flows on the ground surface.