Back in the day when the breed still performed the job for which it was bred, Otterhounds were used with small terriers to hunt otters along the banks of ponds and rivers. The Terriers flushed the otter from its den, and when the otter ran to the water, the Otterhound took over. The Otterhound’s nose is so sensitive that he can follow not only the “wash” (the scent of the otter in the water), but also the “drag,” the trail of the otter on land. The dogs have been known to stay on 12-hour-old trails and to swim and wade as far as 20 miles in a day. From DogTime.com 

Otterhounds by Maud Earl available here