Curvilinear distance (along thalweg) from the reference location to the upstream and downstream reach boundaries, follows this convention:
If either boundary is upstream from the reference location, its value is negative; otherwise, its value is positive. When this convention is followed, the "Curvilinear distance to downstream boundary" MINUS "Curvilinear distance to upstream boundary" = "Curvilinear Reach length"
An illustration of this convention. The stars represent reference locations, in three examples, A, B, C.