A river or stream seeks comfort over its own bed. It scours its channel, wears down obstacles, carries sand grains and boulders drawn from its outback, runs its course with the sheer weight of its will. By erosion and deposition a river modiﬁes the slope of its bed until its rate of ﬂow can transport its load efﬁciently. A river or stream that reaches such equilibrium is called a graded stream. A graded stream is a somewhat theoretical stream—explained by ﬂuid mechanics, felt as mystery. Flow is never perfectly even; interruptions and changes abound. Some stretches of the same stream may ﬂow at grade, while others do not. Flood, drought, rock that resists, sandbars that succumb—ﬂowing water constantly responds to variables. Thus, equilibrium is a dynamic rather than steady balance of energy and resistance—gravity as a ﬂuid medium, reading the land it traverses.