I always bring both. Everyone takes credit cards (NPS, stores, restaurants, etc.). But if their link to the mainland is down they might not be able to run a card. This year at the store it took someone about 5 mins to get a card transaction to go through--I was glad I had cash. Also bring some smaller bills, $20 max.
79: worked RH, 01: BI-DB-RH, 02: MC-LR-WL-CH, 05: MI-CI-MB-DF-RH-TM-RH, 09: MC-BI-DN-RH, 11: WC-HC-WC, 12: MC-CB-HL-TH, 13: RH-PI, 14: BI-ML-CI-CH-MB, 16: RH-CI-TI-RH, 17: WI-IM-SB-FL-WC, 18: MC-PC-BI-DB-RH-DF