Sentence:  The [fishing] technique was to clear the channel downstream from the lagoon for about twenty to forty yards, but not all the way to the salt water. A small "bridge" was left at the lower end of the lagoon. When holding pits had been dug near the end of the <WSD>re-trenched</WSD> channel and everyone was ready, the "bridge" was dug out and the outrush of water brought hundreds of fish along with it. 
Meaning of the word marked with <WSD> tags:  To dig or redig a trench where one already exists.