Ideally: cold in the fridge. Seem to last longest and preserve the flavor best. In any case, in darkness! Lasts years.
Less ideal: Cool, dark, reduced temperature fluctuations. Lasts a year.
OK: On the shelf in the house, in a not-bright room, or covered from light. Lasts many months.
Worst: Direct sunlight, wild fluctuations in temperature, very warm. Lasts a couple months, but might taste not-great after just a month.
How warm is the river water now?
I am basing this off commercial beer. The assumption here is that whatever is in the bottle is just beer and CO2, no bacteria or other spoilage elements.