TopoFlow

Puzzle #—

— grid

7.5-minute series

How it rains

Every cell drains to one neighbor. Water never loops; every drop eventually leaves the grid. Fill in every cell.

Numbers around the edge say how many cells' water leaves the grid through that border. 0 means nothing exits there.
Numbers in cells say how many cells' water passes through that cell, counting its own drop. A 1 is a hilltop: nothing flows into it.
Pieces with a dashed outline are already surveyed. Their whole shape is correct, including every stream that joins them.
Every stream starts at a pond (a cell nothing flows into). Small arrows show which way each cell drains.

Tap a cell to turn which way it drains. Drag across cells to trace a stream, and drag off the edge to send it out of the grid. Right-click clears a cell.