the lake was drained away. The flat floor, spread smooth with
lake-laid silts, is still a plain, to the eye as level as the sea.
Across it the Red River of the North and its branches run in
narrow, ditch-like channels, steep-sided and shallow, not
exceeding sixty feet in depth, their gradients differing little
from the general slopes of the region. The trunk streams have but
few tributaries; the river system, like a sapling with few limbs,
is still undeveloped. Along the banks of the trunk streams short
gullies are slowly lengthening headwards, like growing twigs which
are sometime to become large branches.

The flat interstream areas are as yet but little scored by
drainage lines, and in wet weather water lingers in ponds in any
initial depressions on the plain.

CONTOURS. In order to read the topographic maps of the text-book
and the laboratory the student should know that contours are lines
drawn on maps to represent relief, all points on any given contour
being of equal height above sea level. The CONTOUR INTERVAL is the
uniform vertical distance between two adjacent contours and varies
on different maps.

To express regions of faint relief a contour interval of ten or
twenty feet is commonly selected; while in mountainous regions a
contour interval of two hundred and fifty, five hundred, or even
one thousand feet may be necessary in order that the contours may
not be too crowded for easy reading.

Whether a river begins its life on a lake plain, as in the example
just cited, or upon a coastal plain lifted from beneath the sea or