A Viking crew rows far up a river and puts ashore on a bank where there is no harbour at all. Which feature of their ship makes this possible?

Explanation

A hull that draws little water can go where deeper ships run aground, which is exactly why such crews could work far upstream and step ashore on an open bank instead of hunting for a port. Carrying the ship overland did happen on some routes, but that answer describes something the crew did on land and says nothing about the feature that lets a vessel float in shallow water.