code: def rivermap_update(self, river, water_flow, rivermap, precipitations):
          
  
          isSeed = True
          px, py = (0, 0)
          for x, y in river:
              if isSeed:
                  rivermap[y, x] = water_flow[y, x]
                  isSeed = False
              else:
                  rivermap[y, x] = precipitations[y, x] + rivermap[py, px]
              px, py = x, y
docs: Update the rivermap with the rainfall that is to become
          the waterflow