As the snow fa’s in the river
    A moment white, then lost forever,

sang Burns, in illustrating the fleeting character of human pleasure.  The first snowflakes that fall into the Sierra rivers vanish thus suddenly; but in great storms, when the temperature is low, the abundance of the snow at length chills the water nearly to the freezing-point, and then, of course, it ceases to melt and consume the snow so suddenly.  The falling flakes and crystals form, cloud-like masses of blue sludge, which are swept forward with the current and carried down to warmer climates many miles distant, while some are lodged against logs and rocks and projecting points of the banks, and last for days, piled high above the level of the water, and show white again, instead of being at once “lost forever,” while the rivers themselves are at length lost for months during the snowy period.  The snow is first built out from the banks in bossy, over-curling drifts, compacting and cementing until the streams are spanned.  They then flow in the dark beneath a continuous covering across the snowy zone, which is about thirty miles wide.  All the Sierra rivers and their tributaries in these high regions are thus lost every winter, as if another glacial period had come on.  Not a drop of running water is to be seen excepting at a few points where large falls occur, though the rush and rumble of the heavier currents may still be heard.  Toward spring, when the weather is warm during the day and frosty at night, repeated thawing and freezing and new layers of snow render the bridging-masses dense and firm, so that one may safely walk across the streams, or even lead a horse across them without danger of falling through.  In June the thinnest parts of the winter ceiling, and those most exposed to sunshine, begin to give way, forming dark, rugged-edged, pit-like sinks, at the bottom of which the rushing water may be seen.  At the end of June only here and there may the mountaineer find a secure snow-bridge.  The most lasting of the winter bridges, thawing from below as well as from above, because of warm currents of air passing through the tunnels, are strikingly arched and sculptured; and by the occasional freezing of the oozing, dripping water of the ceiling they become brightly and picturesquely icy.  In some of the reaches, where there is a free margin, we may walk through them.  Small skylights appearing here and there, these tunnels are not very dark.  The roaring river fills all the arching way with impressively loud reverberating music, which is sweetened at times by the ouzel, a bird that is not afraid to go wherever a stream may go, and to sing wherever a stream sings.