def draw_rivers_on_image(world, target, factor=1):
    """Draw only the rivers, it expect the background to be in place
    """
    for y in range(world.height):
        for x in range(world.width):
            if world.is_land((x, y)) and world.river_map[y, x] > 0.0:
                for dx in range(factor):
                    for dy in range(factor):
                        target.set_pixel(x * factor + dx, y * factor + dy,
                            (0, 0, 128, 255))
            if world.is_land((x, y)) and world.lake_map[y, x] != 0:
                for dx in range(factor):
                    for dy in range(factor):
                        target.set_pixel(x * factor + dx, y * factor + dy,
                            (0, 100, 128, 255))
