Complex sentence: There is only 8 feet of headroom under the bridge, which carries the A149 road, and the river is only three feet deep at this point, making it only passable by the smallest of pleasure craft.
Split into simpler sentences: There is only 8 feet of headroom under the bridge which carries the A149 road over the river. The river is only three feet deep at this point also making it only passable by the smallest of pleasure craft.