A flooded river threatens to sweep away a bridge construction project. Ranger Corey Stuart and Lassie brave the raging waters in a small boat to construct a log chain barrier and control the flow of water and logs.

Lassie befriend Patricia Prescott, who refers to herself as ""Walden,"" a free-spirited teenage girl, who has left home to live in the wilderness like her idol, Henry David Thoreau, after the young woman cleans up a picnic area. But both of them are endangered by aftershocks from an offshore earthquake.

While Bob and Dan are scouting out the rapids for a ""whitewater trail"" for tourists, Lassie finds a baby raccoon that has been poisoned by ""pink snow""--the same material she has already eaten. The only way to get her help--a trip down the worst of the rapids.