The Naknek River watershed is the largest in the park and preserve. Notable streams and rivers in this watershed includes American Creek, Brooks River, Headwaters Creek, Idavain Creek, Margot Creek, Rainbow River, Savonoski River, and Ukak River. Water from Naknek Lake drains into the Naknek River which flows west into Bristol Bay and the Bering Sea. All of Katmai National Preserve drains west through Alagnak and Nonvianuk rivers into the Kvichak River and then into Bristol Bay.