Sand Rivers, by Peter Matthiessen, with photographs by Hugo van Lawick. New York: Bantam Books. 224 pp. $11.95.

The author of the award-winning ''The Snow Leopard'' now takes us on a 1979 safari to one of Africa's largest and least-known game preserves, the Selous, in southern Tanzania, to view animals in one of their last wilderness strongholds. ''Sand Rivers'' is a day-to-day chronicle of what a major jungle is like and how man and beast survive there. One learns minute details about the wildlife, including elephant, hippo, and water buffalo observed at close range. As ever, rivers and pools become the focal point for thirsty animals, and the sand rivers , those with little or no water running in them, provide the pathways for travel into the heart of the jungle.