Only a very small portion of the water derived from the melting ice and snow of Shasta flows down its flanks on the surface.  Probably ninety-nine per cent of it is at once absorbed and drained away beneath the porous lava-folds of the mountain to gush forth, filtered and pure, in the form of immense springs, so large, some of them, that they give birth to rivers that start on their journey beneath the sun, full-grown and perfect without any childhood.  Thus the Shasta River issues from a large lake-like spring in Shasta Valley, and about two thirds of the volume of the McCloud gushes forth in a grand spring on the east side of the mountain, a few miles back from its immediate base.

To find the big spring of the McCloud, or “Mud Glacier,” which you will know by its size (it being the largest on the east side), you make your way through sunny, parklike woods of yellow pine, and a shaggy growth of chaparral, and come in a few hours to the river flowing in a gorge of moderate depth, cut abruptly down into the lava plain.  Should the volume of the stream where you strike it seem small, then you will know that you are above the spring; if large, nearly equal to its volume at its confluence with the Pitt River, then you are below it; and in either case have only to follow the river up or down until you come to it.

Under certain conditions you may hear the roar of the water rushing from the rock at a distance of half a mile, or even more; or you may not hear it until within a few rods.  It comes in a grand, eager gush from a horizontal seam in the face of the wall of the river gorge in the form of a partially interrupted sheet nearly seventy-five yards in width, and at a height above the riverbed of about forty feet, as nearly as I could make out without the means of exact measurement.  For about fifty yards this flat current is in one unbroken sheet, and flows in a lacework of plashing, upleaping spray over boulders that are clad in green silky algae and water mosses to meet the smaller part of the river, which takes its rise farther up.  Joining the river at right angles to its course, it at once swells its volume to three times its size above the spring.

The vivid green of the boulders beneath the water is very striking, and colors the entire stream with the exception of the portions broken into foam.  The color is chiefly due to a species of algae which seems common in springs of this sort.  That any kind of plant can hold on and grow beneath the wear of so boisterous a current seems truly wonderful, even after taking into consideration the freedom of the water from cutting drift, and the constance of its volume and temperature throughout the year.  The temperature is about 45 degrees, and the height of the river above the sea is here about three thousand feet.  Asplenium, epilobium, heuchera, hazel, dogwood, and alder make a luxurious fringe and setting; and the forests of Douglas spruce along the banks are the finest I have ever seen in the Sierra.