Forests as Reservoirs

Perhaps half of the major river systems of the West comes from melting snowpacks. These snowpacks act as huge reservoirs, "spilling" their water over long periods in the spring and in the early summer. Research in forest practices has indicated that we can manage the snowpack reservoirs, both to yield more water and to improve the time of delivery to meet our water needs.

Several studies have compared the effects of different types of logging, such as selective cutting, strip cutting, block cutting, and sequential cutting, on snow accumulation and melt and on soil moisture losses. These studies furnish clues to relative effectiveness of management techniques in improving water yield from snow.

Strip cutting has some advantages over block cutting, in part because the narrow clear-cut strip retains snow for a longer period than the clear-cut larger block, and so can deliver more water later in the spring.

The full effectiveness of strip cutting cannot be evaluated by simple comparison of the amount of snow in the strip and in the adjacent forest. Analyses of more than fifty-eight snow courses in the central Sierra of California have shown that increases in accumulated snow resulting from strip cutting occur not only in the cut opening but also in the adjacent forest. An experimental cutting in the Tahoe National Forest, near Yuba Pass, California, illustrates this point.

The Yuba Pass cutting consisted of five east-west strips, 0.6 mile long, 135 feet wide, and 400 feet apart. The cutting permitted comparison of the total snow accumulation and rate of snow melt under five different treatments: slash piled strips, slash lopped strips, strips with small trees left, strips with slash dozed downhill, and wall-and-step strips. The last type of treatment (illustrated) achieved by far the largest snow accumulation.

Cutting was in an old-growth red fir stand on a 15 percent north slope at an elevation of 6,700 feet. The snowpack was measured at some 400 points on April 24, 1963 —about the time of the maximum pack—and again on May 21, when about 60 per cent of the pack had melted. The snow in the center of uncut forest averaged 19 inches of water equivalent on April 24, and 7.7 inches on May 21. Increases in the snow associated with the wall-and-step cut strip amounted to 18.9 inches of water, and increases for the whole forest measured 4.7 inches of snow water. The treatment increased the total snow accumulation by 25 percent for the forest as a whole. Seventy per cent of the forest's increase persisted as delayed melt until May 21.

An increase of this proportion is important enough to encourage management of these high-elevation forests for water yield. In view of later reports from forest officers indicating "little or no blowdown in the severe October storm" and "an economical logging show," it is not too much to hope that treatments of this kind may be feasible.