The best time to get this cache is when the river is nearly dry, which happens rarely.
Do not attempt when the river is high or fast.
Do not attempt when the ice is thin or breaking up. Remember that ponds freeze before lakes and lakes freeze before rivers and streams, which never freeze completely. Near the cache in the dead of winter you can find open water, or worse yet, thin ice covered by snow which insulates it. Even a slight melt may cause the ice to heave and buckle. Prolonged draining will cause it to cave. All these phenomena have recently occurred near the cache. Snowshoes or cross-country skis are recommended to distribute your weight. Partners should be separated and tied to one another. If solo, tether yourself to a tree on the east bank. Use your poles to check for thin ice and fissures. Fear the ice.
11/18/17. The mainland to the east is now public and there's a treacherous log jamb connected to the island. I still would not recommend attempting this solo.