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001521t volcanic rock overlain by a very thin soil mantle. The stock ponds are characteristically shallow with a maximum depth of about 5 feet. Filling occurs each spring from snowmelt, but prior to treatment the water o;;eeped away in from 1 to 3 weeks. For treatment the pond floor was covered with from 1/2 to 1 inch of impure bentonite secured from a quarry'about 50 miles distant. On wetting the clay expanded to a layer about 3 inches thic.k. This practically eliminated seepage in the pond and the water remained in storage until lost by evaporation and other causes. ,-..,....'", The treatment used in this instance required from 75 to 125 cubic yards of bentonite per ,acre of stock pond floor. It was obtained at a cost of about $5 per cubic yard spread. Although expensive, the treatment is justified On the basis of large benefits obtained. Occasional renewal of the blanket is needed to compensate for the bentonite lost by wind during the summer months when the stock pond is dry. Because of this loss dur- ing dry periods, the bentonite treatment is most effective in ponds that retain water yearlong. _::;- ~~; . .~ ~ The suggestionha.s beenadvanced that providing a compacted layer of soil over the entire stock pond floor at the time of construction would be beneficial in reducing seepage., Doubtless such treatment would result in substantial saving of water in many localities, although data based on actual measurements are not available. If written into the specifications and included as a part of the construction contract s.o that it could be carried out at the completion of construction when both water trucks and heavy compacting equipment were at the site, the treatment could be accomplished at a minimum of expense. Since stock ponds are constructed to provide water for livestock for as long as possible, a,h extra charge to reduce seepage losses may well be justified. In cases of excessive seepage that cannot be corrected by the methods described above, use of pla'stic.Jining or impervious pavement may be justified. Plastic of the proper weight and toughness is now available in large sheets which can be joined by cement to make a water- proof lining extending across the stock pond floor. If covered with a minimum of 6 inches of so il and protected from trampling by livestock, the lining should last ,indefinitely. The treatment will be expensive but probably will be less than is involved in abandonment of the reservoir and construction of a new one or development of water by some other means. Use of waterproof pavement for this same purpose may be advantageous and the choice will depend on a number of economic and engineering considerations which should be analyzed before the final treatment is selected. Several organic products, most of them petroleum derivatives, are now being u sed to redu ce seepage in irrigation canals, and appear 34 ",;-