BASELEVEL, GRADE AND PENEPLAIN 95 
stating that one has developed its grade on a faint slope, the 
other on a stronger slope. 
The incapacity of the Platte to deepen its valley leads Gan- 
nett to describe it as an “‘overloaded river( 1900); but this phrase 
is not altogether satisfactory, because it overlooks the fact that 
rivers refuse to be overloaded. A river will most dutifully work 
up to its full capacity; it is ready to increase its capacity by 
increasing its slope through aggrading when necessary (as stated 
below), and thus it may become heavily loaded; but like the 
traditional llama it refuses to carry an overload. Like all 
streams with braided channels, the Platte is well graded, as well 
graded as the typical lower Missouri, although the quantity and 
texture of its load require it to maintain a relatively strong 
slope. 
Baselevel remains fixed all through an uninterrupted cycle: the 
slope of graded streams must vary as the cycle advances.— After a 
river system has attained a maturely graded condition, it will 
maintain a graded condition through all the rest of the undis- 
turbed cycle; but it is important to recognize that the mainte- 
nance of grade, during the very slow changes in volume and load 
that accompany the advance of the cycle, involves an appropriate 
change of slope as well. Instead, therefore, of having to do 
with a fixed control of erosion, such as is found in the general 
baselevel of a region, we have here to do with a slowly, deli- 
cately and elaborately changing equilibrium of river action, 
accompanied by a corresponding change in river slope. For 
example, a large river in a mountain valley may reach grade in 
the early maturity of its region. It will then flow with a rush- 
ing current on a rapidly sloping bed of cobblestones; and it 
may stand hundreds or even thousands of feet above baselevel. 
In the old age of the region, the same river will flow with a slug- 
gish current ona nearly level bed of sand and silt through a 
peneplain, only a few tens or scores of feet above baselevel. 
This is a point that is not generally enough recognized. It 
is too often implied —in the absence of explicit statement to 
the contrary —that when a river is once balanced between 
