Rivers are intrinsically part of their watersheds and valleys. In order to understand what is going on in a river, you need to look at what is going on in the adjacent riparian area and valley floor. There are a number of datasets and tools that delineate watersheds or catchments, but these are not applicable for associating portions of the valley floor with the associated reach or portion of the river. Further, these tools are not designed to work with bespoke river reaches that aren’t hydrologically designated. This document looks at several methods for delineating and associating portions of the valley floor with the adjacent river reach.
We will be using using the Chewuch River in north central Washington State as an example. River Reaches are defined to corresponds with a model framework and assessment and prioritization strategy (https://www.ucsrb.org/science-resources/mapping-library/). The valley floor and defined reaches we are working with are shown below.
Simply dividing the valley floor based on perpendicular divisions based on the river center lines results in breaks that don’t really make sense.
One method for dividing the valley floor into reaches is to generalize the river line using the reach endpoints, then make perpendicular dividing lines at each endpoint. This results in better divisions than the naive approach. Some issues remain when the transects intersect within the valley floor.
Another approach for determining reach breaks is to generate a TIN or triangulated irregular network based on the perimeter of the valley floor and use the closest transect to each reach endpoint as the basis for reach divisions. TIN divisions should be reasonably close to shortest path lengths across the valley floor, and are based on the valley floor perimeter itself rather than the meandering river line.
To make this function, once the boundaries of each triangle are defined, the one nearest each reach endpoint is selected and shifted to cross the reach endpoint. These lines then become the basis for the reach divisions.
A completely separate method is to divide the valley floor into grid cells and assign each cell to the closest reach. This approach handles the bends well and makes intuitive sense for analyses that are considering what area is contributing or interacting based on proximity. The disadvantage of this method is the divisions are not straight lines.
Let’s take a closer look at how the results of the three improved methods. Each of the three methods result in a reasonable but different reach divisions. The grid based method seems most appropriate when running an analysis of how the river is affected by riparian land use. The TIN based and generalized river direction approach are more appropriate when cross-sections are needed (such as for hydraulic modeling or generating a relative elevation map). It looks like the TIN based method is more sensitive to bends in the overall valley width, and less sensitive to random effects due to where the reach divisions are located.