William Clark (1770-1838) was an American explorer. Between 1804 and 1806 he was second-in-command of the Lewis and Clark Expedition from Pittsburgh to the Pacific Coast.
In "Background Blues" Francine has a fantasy in which her ancestor Ma Frensky saves Lewis and Clark from a bear and points them towards the Pacific Ocean.
In the introduction to "Dancing Fools" Arthur imagines Clark setting out without Lewis. Clark falls out of the canoe.