The macrobenthic invertebrates of a canal, created from a tributary of the Warri River in the mangrove swamp of the Niger Delta, Nigeria, were studied before and after dredging. In the canal dredging resulted in at least a 93% decrease in the benthic population while the Margalef's diversity index reduced from 3.8 to 1.4. Of the 15 species identified during the pre-dredging studies, only Nereis operta and Baetis sp. were recovered after dredging. The site 500m downstream of the canal was also slightly impacted, showing a 31% decrease in population. We conclude that the drastic reduction of benthic species is due to the direct destruction of benthic species, larvae and habitat, as well as settling turbidity plumes, reduction of sediment nutrients, physical disturbance and, physiological and toxic stress. To conserve biodiversity it is recommended that mitigation measures should be put in place by stakeholders involved in dredging and that long-term monitoring of dredged canals is carried out to ensure that these measures are effectively implemented.