1const riverSizes = input => {
2  let results = [];
3  input.forEach((row, y) => {
4    row.forEach((cell, x) => {
5      if (input[y][x] === 1) {
6        results.push(checkAdjacent(x, y, input));
7      }
8    });
9  });
10  return results;
11};

1const checkAdjacent = (x, y, input) => {
2  input[y][x] = 0; // mark cell as visited
3  let size = 1;
4
5  [[x + 1, y], [x, y + 1], [x - 1, y], [x, y - 1]].forEach(([i, j]) => {
6    // make sure we don't access a row that doesn't exist
7    // then check to see if we have a river
8    if (input[j] && input[j][i]) {
9      size += checkAdjacent(i, j, input);
10    }
11  });
12
13  return size;
14};