It Started With a Kidnapped Pig...
The Dessarin River is cold, and deep for most of its length and fast flowing in spring. It is navigable as far north as Dancing Falls, at the base of the Lost Peaks. A small boat turning up the Rauvin can paddle and pole as far north as Dead Orc Pass, northeast of Sundabar. You can navigate up the Surbrin almost to its source, or up the Redrun as far as the Citadel of Many Arrows. Heading south and west you can reach Waterdeep via the Dessarin River.
The Dessarin River is home to many silver, troutlike fish that grow to two feet in length. These are called shalass, and they’re highly prized on tables throughout the North. These are best caught in a particular type of fish basket called a cone net. Such baskets are mounted on long, sturdy poles, and they require great strength to hold. Expert fisherfolk use long spears to stab the swimming fish from the bank, or from a raft or boat.
The persistent traveler can also find catfish, coldwater crabs, and small brown fish known as lout. The last are tasty when pan-fried, but you need seven or so to fill the pan. There are also fresh water carnivorous fish, analogous to sharks. The chilly waters of the Dessarin also hold larger and more dangerous life, so beware!
There are three bridges across the giant Dessarin River – Ironford bridge (near Womford and the Bargewright Inn), Stone Bridge north of the Sumber Hills near Beliard, and Zundbridge near Waterdeep – and these are (or will be) detailed in their Wiki entries. Of course, people crossed rivers long before bridges existed in the North, but such crossings can become hazardous, especially in the spring with the waters swollen from the melted snows.