Normal sudoku rules apply. There's a one-cell wide 'river' of orthogonally-connected cells snaking from R9C9 to R1C2 without branching or touching itself orthogonally (but it may touch diagonally). Digits along the 'river' differ by at least 5. All remaining cells are 'land'. 'Bridges' (=) only cross single 'river' cells, and are the sum of the two 'land' cells at each end of the 'bridge' (e.g. if the river cell was a 5, the two land cells could be 2&3 or 1&4). Diagonal 'bridges' only cross 'river' corners. Placing a correct digit will clear fog in its cell, and may also clear fog in other cells.