Particles suspended in the river (such as silt, sediment, or plant material) determine the water's clarity (also called turbidity). Simply put, the higher the turbidity, the murkier the water. High turbidity can diffuse sunlight as it penetrates the water's surface, possibilty even affecting the local flora and fauna.
On the Half Moon, we use a Secchi disk to measure the Hudson River's turbity, although this device would have been an anachronism in 1609. Hudson's voyages predate the invention of the Secchi disk by more than two centuries; in the 17th century, sailors would simply keep an eye on objects floating beneath the water's surface -- an inexact, "rule of thumb" approach.
