Slow moving water, shallow water and excess nutrients will encourage weed growth. Are there upstream farms that would have fertilizer running off into the river? Has someone added a dam downstream to slow it? Has there been a lack of rain so that it is shallow? Or have the banks eroded so that it is wider and shallower?

Generally, you cannot alter a watercourse or interfere with it. Is there a Conservation Authority responsible for your watershed? They will be able to provide you with advice about managing your river and what is allowed. Many times they would welcome measures like adding rocks and trees to the river bank to narrow it and deepen the water.