Reports from the 19th century are intriguing.
Charles M. Skinner, in Myths and Legends of Our Own Land, J.B. Lippincott Co., 1896, II:303, has this very brief and unattributed report of an earlier monster in Rock Lake:
So with Rock Lake, in Washington. A hideous reptile sports about its waters and gulps down everything that it finds in or on them. Only in 1853 a band of Indians, who had fled hither for security against the soldiers, were overtaken by this creature, lashed to death, and eaten.
Source: Spokan Falls, Washington State The Spokan Times, July 24, 1879, p. 3.
[That's the whole thing, says Chuck Flood who contributed this item.]