The Vanishing Shadow — Margaret Sutton

In the first book of this mystery series, Judy Bolton is resigned to spending several boring weeks at her grandparents' farm near Dry Brook Hollow. Her summer suddenly becomes interesting when she overhears some men talking about the Roulsville dam. Although, Judy does not understand the conversation, the men fear that she does and threaten her. Judy also becomes reacquainted with her childhood friend, Peter Dobbs, who gives her a cat that she names Blackberry, and meets the wealthy siblings, Arthur and Lois Farringdon-Pett. As the days pass, Judy learns that the Roulsville dam is cracked and may break during the next rain. The moment of truth arrives when a torrential downpour threatens the dam and Judy must rely on her seemingly cowardly brother Horace to warn the people of Roulsville of the danger. All Judy can do is hope that the dam holds long enough for Horace to warn the townspeople in time.