A tilt in the landscape is called a grade. We know grade viscerally in steep country, and in more subtle ways where the tilt is slight. Water’s “line of desire” is directly down, but the nuances of grade translate this simple urge to myriad expressions of declination and ﬂow in streams, from falls to rapids, to fast water and long reach, to pool and slack. One may “climb the grade,” or “go over the grade” at a low point of the ridgeline. Generally, the term refers to gradient: the climb up, and the going down. The Lewiston Grade in Idaho, bane of truck drivers until furnished with escape ramps, is one spectacular example. In scientiﬁc writing, grade is a term of some complexity, referring to the equilibrium between erosion and deposition in streams—ideas developed in the West.