The blocked lock

An original fiction passage set in the nineteenth century.

At dusk, Abel steered the cargo barge into the canal lock. The keeper pushed against the upper gate, but a fallen branch was wedged beneath it. He reached for the iron winding handle and found that it had cracked at the socket. While the crew secured the barge for the night, Abel climbed onto the bank to seek help at the nearest cottage.

Give two reasons the barge cannot pass through the lock.