further clarify gradient calc description

docs/02_model_access.md

@@ -28,7 +28,7 @@ Natural barriers identified by stakeholders include:
 
 FWA stream network lines hold standardized Z values; each vertex of a stream line holds an associated elevation value derived from the BC Digital Elevation Model. Absolute elevation accuracy is subject to error in the DEM, but all elevations have been processed to ensure *relative* elevation is clean - all streams flow downhill (see the [Technical Specification](ftp://ftp.gdbc.gov.bc.ca/sections/outgoing/bmgs/FWA_Public//Documents/FWA-SegmentElevation.TechnicalSpecification.v0.7.pdf) for details). With these clean Z values, we can confidently calculate a reasonable estimate of the gradient of a stream at any point.
 
-To identify locations where a stream's slope exceeds a given threshold, the model starts at the mouth of a stream (identified by the `blue_line_key`) and iterates through each vertex of the stream flow line.  At each vertex, it calculates the slope of the stream from the vertex to 100m upstream. If the 100m slope exceeds a given threshold (eg 25% for salmon), the point location is recorded as a 'gradient barrier'.
+To identify locations where a stream's slope exceeds a given threshold, the model iterates through each vertex of the stream's flow line (identified by the `blue_line_key`) - starting at the mouth and working up to the headwaters.  At each vertex, the slope is calculated from the vertex to a point 100m upstream. If this 100m slope exceeds a given threshold (eg 25% for salmon), the point location is recorded as a 'gradient barrier'.
 100m was chosen as the interval because:
 
 - 100m is the traditional minimum length used for measuring slope and defining reaches in BC, as per the [Fish Stream Guidebook](https://www2.gov.bc.ca/assets/gov/environment/plants-animals-and-ecosystems/fish-data-information/fishstream.pdf)