No, it's literally a row of spikes jutting out of the sand diagonally. 


[img]https://puu.sh/E3bXD/9b07b39695.png[/img]

The force of the river forces the larger fish to go down it. With no way to go back up, they are forced into the spikes, where they die. 

The smaller fish are able to swim through, but the large fish are impaled.