Characterizing River Channels with GIS
GIS offers a means to characterize many aspects of river channels prior to going into the field. This is a project I assign students in my GIS courses in order to familiarize them with semi-quantitative "metrics" of channels. Check out my other website for many how-to lessons targeted at researchers working in GIS and the Geosciences: www.GIS4Geomorphology.com.
Students utilize Google Earth, Excel, and Illustrator to “stack” various bits of information that characterizes a channel of their choosing. This project does not require ArcGIS skills. The final product is an 11″ x 17″ poster. Click on image below for a printable PDF. The Crazy River is ficticious.
CHOOSE A CHANNEL Use Google Earth to locate an interesting reach of a mid-sized river that flows through mountains and into a flat basin. Look all over the world. 80km to 100km reaches work well. Choose rivers that are not too big, not too small – larger tributaries to regional trunk streams tend to work well. Look for rivers that drain prominent mountain ranges. Look for interesting landscapes in terms of tectonic activity, geology, and climate. The most interesting long profiles tend to record the effects of extreme relief, areas of geologic sameness (all basalt, for example) or highly complex geology, or formerly glaciated/unglaciated terrain, or tectonically active/stable areas.
LONG PROFILE You need a ~100 km long reach of a mid-sized river that runs from mountains to a basin. Use Google Earth and Excel to compile information on 50 placemarks along the channel. Space them roughly evenly. Save a .kmz containing all Riverpoints. A “segment” runs between two adjacent Riverpoints (i.e., 49 reaches). Compile data in Excel, create the poster in Illustrator.
ELEVATION Use Google Earth to find river surface elevation at each of 50 Riverpoints.
DOWNSTREAM DISTANCE Use Google Earth’s measurement tool to measure the along-channel distances between pairs of Riverpoints. Make sure to zoom in and click carefully along thalwegs. Record distances in spreadsheet.
SLOPE Use Excel to calculate slope (rise/run) for each segment.
HACK’S SL INDEX Use Excel to calculate Hack’s SL Index for each segment.
SINUOSITY Use Excel to calculate channel sinuosity for each segment. Sinuosity is downstream distance measured along the channel between adjacent Riverpoints divided by straight-line point to point distance. Record value next to the downstream point in your spreadsheet.
GEOLOGY Locate a bedrock geologic map for the area cut by your channel. Record the major rock types, formation name, age, and where the contacts between units cross your profile. Locate contact crossings and any faults on your profile.
CONCAVITY Most long profiles will contain at least one prominent knickpoint (conspicuous convexity). Locate knickpoints on profile. Calculate concavity via θ method or via SCI method for the channel segment above and below the most prominent knickpoint in your profile. See the Channel Concavity lesson for 3 different methods and how-to instructions, including MatLab. Create separate Excel charts for concavity as shown in the example. Include the charts on your final figure.
KNICKPOINTS Place dots on your profile at knickpoints. Write a short caption to explain the presence of your prominent knickpoint. Include caption as a text box in final poster.
GEOGRAPHIC FEATURES / REFERNCE POINTS ALONG PROFILE You need about 10 named geographic locations distributed along your profile. Bridges, villages, road crossings, faults, major confluences, etc. Record on spreadsheet as a note that refers to nearby Riverpoint(s). These help orient the reader.
RIVER STYLE / PLANFORM GEOMETRY / RIVER CHARACTER Break your channel into its various styles using clues from textbooks by Brierley and Fryirs (amongst others). Most channels will break out into several different styles along their length. My rule of thumb is to shoot for 5-10 river style breaks in a 100 km reach. In reaches where frequent style changes occur, create a composite style category to characterize the pattern rather than one specific style or another. Record the style breaks in a spreadsheet. River style will be illustrated graphically in a final figure. Include brief notes on the following for each section (see example figure at top of this post).
– Nature of lateral confinement – Sinuosity – Location and density of bars and islands – Braiding/Meandering/Straight/Anastomosing forms – Valley setting and local landforms – Likely grainsize of bed and banks (or field measurements) – Avulsive behavior interpretation
REPRESENTATIVE CROSS SECTIONS Select 3-5 locations along your profile for cross sections (topographic profiles). Sections should be of similar length and highlight topographic variation of the valley. Construct a summary figure showing the topographic profiles stacked atop one another and at the same scale (x = distance along section, y = elevation). Not shown on example figure.
CHANNEL WIDTH Use Google Earth measure tool to find flowing channel width (bankfull) or active channel width (braided streams). This is not going to be super accurate, but at the scale of the project, it will be helpful in characterizing segments.
VALLEY WIDTH Use Google Earth measure tool find width of valley. I define this as distance between “confining topographic landforms” (bedrock walls, higher terraces, alluvial fan toes, etc.). Not an exact science, but useful in visualizing the geometry of the valley bottom/floodplain in plan view.
RIDGELINE PROFILE At each Riverpoint, find the highest point within about 2 km, measured perpendicular to the channel. This idea is to characterize the height of cliffs next to the channel. Usually one ridgline profile is sufficient (River Right or River Left).
OTHER THINGS YOU COULD ADD...
– Terrace elevations (tread spot elevations or segments). – Number of water wells within a short distance from channel; project well location to channel. – Landuse along channel. – Counts of bars (usually not stable, mobile sediment) and/or islands (stable, vegetated, bedrock). – Field observations (bank character, bed grainsize, bars, human modifications, dams). – Vegetation types along channel. – Valley Height to Width ratio for entire reach or between knickpoints or other breaks. – Flow accumulation (use DEM and ArcGIS Hydrology tools). – Mapped info or data from reports authored by others (water quality, irrigation withdrawals, point sources, industrial sites, etc.).