How you can carry out depends on the depth, flow and whether it is tidal.
We work on the Thames in central London every month and a steel workboat skippered by a team of Thames waterman is essential.
I would think that some slow flowing shallow rivers could be cleaned without the need of a boat, findout about the river when in the month is lowest tide and highest tide and what time frame do you have between high and low water.
Check all the safety aspects before attempting the job.
If it is a tidal river can you work from the foreshore at low tide?