The accumulated flow crossing a given edge of a cell: the flow of whichever
cell is upstream of that edge. Because both cells sharing the edge resolve
to the same upstream cell, they always agree — this is what keeps channel
width continuous across cell (and chunk) borders.
Falls back to the cell's own flow for edges without a paired upstream
(sources, estuary mouths, dangling half-edges).
The accumulated flow crossing a given edge of a cell: the flow of whichever cell is upstream of that edge. Because both cells sharing the edge resolve to the same upstream cell, they always agree — this is what keeps channel width continuous across cell (and chunk) borders.
Falls back to the cell's own flow for edges without a paired upstream (sources, estuary mouths, dangling half-edges).