Comprehensive Notes on Map Reading, PLSS, and Watersheds

Public Land Survey System (PLSS)

• Lab this week involves map reading with topographic maps.
• The PLSS is a primary focus, involving township, range, and section.
• This system describes an area rather than a specific point, dating back to 1785 when Congress needed a system to describe land for sale or distribution to settlers.
• The system drapes a grid network over space, referring to areas that are one or six square miles.

Origin and Structure

• The American West, at the time of the survey's inception, was largely a blank slate for settler cartographers.
• The system uses anchor points like a hill outside Portland, Oregon, and numbers increase with distance from this point.
• It resembles a spreadsheet with rows (townships) and columns (ranges).
• Township numbers increase north, and range numbers increase east.
• Location is described using township and range numbers (e.g., Township 13 North, Range 3 East).
• Each township is 6 miles by 6 miles, totaling 36 square miles.

Sections

• Townships are divided into 36 sections, each one square mile.
• Sections are numbered starting in the Northeast corner, proceeding back and forth as if plowing a field.
• The numbering pattern goes from 1 to 6 in the first row, then 7 to 12 in the second row, and so on, ending with 31 to 36.

Practical Implications and Errors

• The PLSS was surveyed on foot, leading to human errors that remain today due to its physical inscription in space.
• Section lines may not perfectly connect, and correcting these errors could disrupt existing property lines.
• Each section contains 640 acres.
• Land disposal acts, like the Desert Land Act, distributed land in these units (e.g., 640 acres under the Desert Land Act, quarter sections of 160 acres under the Homestead Act).
• Sections can be further subdivided into quarters (160 acres) or smaller units.

Notation

• Locations are written from specific to general: Section, Township, Range (e.g., Northwest Corner Section 23, Township 4 North, Range 40 East).
• When reading notations, it's best to start with the township and range to orient yourself to the larger area, then find the specific section within that township.

Map Reading Tips

• PLSS information is printed in red ink on USGS topo maps.
• Maps show township and range information in the margins.
• Thin red lines overlaid on the map indicate the section grid.
• USGS maps are based on latitude and longitude (15-minute or 7.5-minute series) and do not align perfectly with the PLSS grid.
• The map area contains whatever portion of the PLSS falls within that geographic division.
• To interpret the grid, look for discontinuities in section numbers to identify the boundaries between townships and ranges.
• Use the map margins to determine township and range numbers, then find the corresponding section within the grid.

Watersheds and Streamflow

• Watersheds and drainage basins are geographic areas bounded by high elevation topography.
• Precipitation within a watershed flows to the lowest point and drains out to sea via a stream network.
• Watersheds act as catchment areas, collecting precipitation and channeling it into tributaries and main streams.

Precipitation and Runoff

• Vegetative cover can intercept precipitation before it reaches the surface.
• Water that reaches the surface either runs over the land (overland flow) or soaks into the soil.
• Overland flow occurs when soils are saturated or resistant to water.
• Water that soaks into the soil is captured by plant roots and evapotranspirated back into the atmosphere.
• Some water percolates through the soil and forms groundwater.
• Water flows as either surface flow or through flow, eventually reaching streams.

Stream Discharge

• Stream discharge is the volume of water flowing downstream past a given point, measured in cubic meters per second or cubic feet per second (CFS).
• USGS operates stream gauges to measure discharge, with some gauges having long-term data (e.g., up to 80 years).
• A cubic foot per second (CFS) is about the volume of a basketball flowing by each second.
• Hydrographs represent measurements of discharge over time.

Hydrographs

• Spikes on a hydrograph indicate precipitation events.
• Base flow represents the discharge expected absent specific precipitation events (e.g., groundwater inputs, melting snow).

Factors Affecting Streamflow

• Groundwater feeds into streams, especially during dry periods.
• Watersheds respond differently to precipitation based on geology, land use, topography, and soil types.
• Some watersheds are slow to respond, while others are flashy, showing rapid spikes in discharge after rain.

Measuring Discharge:

Discharge = Width * Depth * Velocity

• USGS surveys stream channels to relate water height (stage) to discharge measurements.
• They measure channel width, depth, and water velocity to create these relationships.
• Stream channels are divided into increments (e.g., one-foot intervals), and depth and velocity are measured at each increment.

Hydrograph Analysis

• There is a lag between precipitation events and when the water shows up downstream.
• The rising limb of the hydrograph indicates the increase in discharge after a precipitation event.
• Peak discharge occurs after the lag, and then the hydrograph recedes.

Land Use Impacts

• Land use changes can significantly affect discharge patterns.
• Impervious surfaces (roofs, sidewalks, parking lots) reduce the area's capacity to absorb water, leading to increased runoff.
• Urbanization increases the flashiness of stream discharge.

Examples:

• Mercer Creek (urbanized) shows a rapid response, while Nuwakum Creek (less developed) has a slower response with a more spread-out peak.

Other Factors:

• Recently burned areas show increased flashy storm events.
• Dams operated for hydropower can alter stream discharge patterns to match electricity demand rather than natural precipitation events.