Dredging of these shallows was needed to make the river navigable to Harrison Lake, at the north end of which the townsite of Port Douglas was established as the port for the Douglas Road to Lillooet in the upper Fraser Canyon, in order to bypass hostile territory in the lower Canyon (see Fraser Canyon War).

There are also small rapids and difficult water in the first stretch of the river downstream from Harrison Lake, which is a forested canyon.

Below the confluence of the Chehalis River, at the bridge between Chehalis and the Municipality of Kent, the river widens into a wide backwater, Harrison Bay, then from there joins the Fraser.

In addition to the aforementioned bridge (Hwy 7) the Harrison is also crossed by the mainline of the Canadian Pacific Railway at the site of the former milltown of Harrison Mills and the heritage post office and store preserved as Kilby Provincial Park.

The river's mean annual discharge according to this long-operating gauge is 442 cubic metres per second (15,600 cu ft/s).