Unit 3 - Watersheds Overview
Lecture 3a - Watershed Analysis Overview & Getting the GIS Layers
As with all GIS application, first you must think of what is it you do in your line of work or what problems are you trying to solve. For the watershed analysis that would often be protecting, restoring or maintaining the health of a stream. A complex set of factors influence its condition.
Think about your local stream or perhaps one that you played in as a kid. My local creek was an enchanting place, with vines growing up massive trees lined along the banks. It had clear water running over gravel beds, and I can remember wading in it, looking down on fish in deep shimmering pools.
Today, I understand how certain conditions had to exist for fish to be in that stream, such as a lush riparian canopy to help shade the water (thus keeping water temperatures cool). I also remember looking up at long stalks of what I called "bamboo," which I've come to learn is generally a bad thing. It's an invasive plant (called Arundo donax) that slurps up large quantities of water. These are just two simple factors, out of perhaps hundreds, when analyzing stream health.
Others factor might be the stream hydrology, the mechanics and physics of a stream as it meanders or incises its way through the landscape. Or you can sample the water chemistry and look at stream bed conditions… is it clean gravel or filled with sediment? Or one can sample the aquatic bugs in the stream (believe it or not) to indicate the health of a stream in that reach. GIS is well-suited to keep track of these conditions because we can build a set of layers and tables that account for these factors.
But first, let's start with what we call a "base map." These GIS layers are often found in all GIS maps, such as roads (of all sizes), railroads, political boundaries (such as county and city limits), landmarks, hydrology (such as streams and lakes), airports, schools, etc. If you have ever looked at a U.S. Geological Survey (USGS) topographic map, you'll see these features.
Lecture 3b - Introduction to Base Maps
Base maps consist of physical and cultural features, the most basic types to find ones geographic location.
We have absolute locations such as an address, parcel
number, a grid system coordinates (of which there are many), or something as basic as an intersection of two line features, such as Rocklin Road and Secret Ravine. Often salmon are spotted at this location, so we want a reference point. Using Global Positioning System (GPS) units are of great use here when we need to pinpoint a location (discussed later in the class).
There are also relative locations. Those same salmon spotted above are also just downstream of the Sierra College nature trail. Likewise, if we were to take a water chemistry sample, we would want to know if we are next to a waste water treatment plant. A small powerline corridor passes above the nature trail, which would also be of use in determining relative location.
Other important locations are political boundaries, such as the Rocklin city limits, sphere of influence (a slightly larger area providing basic services), community areas, watershed perimeter (an area drained by a whole and connected stream system), a flood plain, etc. In regard to relative location, we would also like to know geographic features such as airport or airbases, such as McClennan A.F.B. or schools, such as Sierra College. Other physical features might be contour lines, elevation points, and vegetated. Very little distinction is made in the vegetation cover type, however, which is left for thematic layers. Often an aerial photo is considered part of the base map because so much information can be derived from it, such as identifying buildings, landcover features, and prominent trees not shown on the base map.
Lecture 3c - Watershed Analysis / Base Maps Layers
List of base map layers developed for the Dry Creek Conservancy (DCC)
(some layers may be used for thematic layers* and/or a different base map version**):
Aerial Photographs - essential for background image, matching and creating main base map features and providing other info. like buildings, landcover, and many features not digitized in base map.
Topography & Contours - hillshade needed to determine the direction of water flow (as defined by a watershed), slope, flooding, etc. **
Hydrology - subdivided into different classes of streams (intermittent, perennial) and other major water bodies (reservoirs, natural lakes, ponds, etc.) Canals are also included. Used for general reference, wildlife habitat corridors (fish, birds, beaver, deer, etc.), water supply and quality, flooding, recreation, etc.
Vegetation - no specific details. Indicates forests, shrubland, wetlands, and agriculture. (Note: for the DCC, this is only shown on the Digital Raster Graph, which is a scanned USGS quadrangle / standard 1:24,000 topo map.)
Major Boundaries - watershed or drainage boundary, subsheds (smaller drainages), counties, cities, special districts (e.g. flood, school, etc.)
Highways & Roads - reference for transportation routes and relative location. Can also use for erosion studies later.
Railroads - major tracks (which may or may not be needed)
Airports / Airbases - large and small airstrips, mainly used as reference points, such as McClellan A.F.B.
Major Landmarks - museums, schools, mines, historic sites, etc.
Elevation Points - along with labels for major peaks, useful for relative elevation references.
Significant Dams - often associated with reservoirs (above) and open space. Useful for assessment of anadromous fish passage, flooding, etc.
Historic Mines - shown on the USGS topological maps. Need to verify the current existence and condition. *
Public Land Survey System - sections of lands conforming to Township & Range, with each section conforming to USGS topographic quadrangle maps.
Public Land Ownership & Open Space - all non-private ownership by agency (Federal, State & Local levels)*
Parcels - not essential, yet useful for large-scale maps, such as locating specific property owners for notifications, proximity to stream, etc.*
Private Open Space and Common - mostly golf courses, but also other nature preserves, conservation easements, subdivision common lands, railroad lands, utility companies, etc.
Hint: also browse reference article below before taking Quiz 1
USGS Digital Base Map Data - Where to Get It,
How to Use It
- Look at what is a DOQ, DRG, DLGs, etc.
- Know specific details about DLGs
- Use as reference in the future