Streams: longitudinal analysis
Where ordering describes the structure of a network, longitudinal analysis describes a stream along its course: how its elevation falls from source to mouth, which branch is the main one, how much channel lies upstream of a given point.
These five tools take the stream raster and the D8 pointer produced earlier in the toolbox.
Start here
Section titled “Start here”Long Profile
Section titled “Long Profile”Plots elevation against distance along the stream, from the headwaters down to the outlet. The result is an HTML chart, downloaded to your computer.
The shape of that curve is the subject of a whole branch of geomorphology. A smooth concave profile is a channel in equilibrium. A step, a knickpoint, marks something that interrupts it: a resistant rock band, a fault, a former base level, sometimes a dam.
Long Profile From Points does the same along a set of points you supply, which is how you profile a specific reach rather than the whole network.
Find Main Stem
Section titled “Find Main Stem”Identifies the principal channel through the network, the one a river takes its name from. Everything else is a tributary.
Useful for cartography, for reporting along a named river, and as a preliminary step to a long profile, since profiling the main stem is usually what you want.
Length Of Upstream Channels
Section titled “Length Of Upstream Channels”Gives, for each cell of the network, the total length of channel upstream of it. Drainage density and network development follow directly from it.
Farthest Channel Head gives the distance to the most distant source, which is the classic measure of the hydraulic length of a catchment and an input to time of concentration estimates.
All the tools
Section titled “All the tools”| Tool | What it does |
|---|---|
| Long Profile | Elevation against distance along the stream network, as an HTML chart. |
| Long Profile From Points | The same profile along a set of points you supply. |
| Find Main Stem | Identifies the principal channel through the network. |
| Length Of Upstream Channels | Total channel length upstream of each point. |
| Farthest Channel Head | Distance to the most distant channel head, the hydraulic length. |
Profiles are downloads. The two profile tools produce HTML documents rather than map layers.
Conditioning shows up here. A long profile is the most revealing quality check of your whole workflow. If it comes back with steps and flats that do not exist on the ground, the elevation model was over filled. Go back to Depressions & Storage and prefer breaching.