When exercising your dog, follow any local guidance or on-site signage to ensure that your pet isn't disturbing wildlife, or stressing the grazing animals that create and maintain habitat.
When visiting rivers, keep your dog out of the water if they have recently been treated with a topical flea treatment. These are essentially pesticides which can leach into the water and cause harm to aquatic wildlife, like dragonfly, beetle and mayfly larvae.
If the site you're visiting allows dogs to swim, encourage your dog to stay in the water rather than climb in and out repeatedly, especially if there are bankside plants at risk of being trampled.
Bare soil erodes easily, releasing sediment into the river which is harmful to the insects and fish that live there. You might not know that severe erosion that even lead to bank collapse! Use areas of hard standing if there are any - some sites even have specially-installed 'dog dips' providing access points to the water which don't contribute to erosion.
The British Veterinary Association also have advice on responsible use of parasiticides on their website.