What we could do to address water quality and habitat issues:
* Protect all the waterways where manoomin still grows.
* Restore as many as possible of the former ones by creating protected zones around them so the water will be clean and behave as needed for the wild rice to grow.
What we could do to address growing season issues:
* Start throwing seeds into lakes at the northern edge of its habitat, to see it will creep north with the shifting temperatures. New territory may become suitable as old territory becomes unsuitable. This will not help the tribes who relied on it, as they are now stuck with a tiny fraction of their old homeland, but it might just save the rice.
And one more that no white person is going to mention:
* Pray over it. Track down all the old rituals and customs and restore as many of them as possible. Because spirit plants and animals can be damn finicky -- if they don't feel appreciated, some of them will pack up and leave everyone to starve. You had instructions for making it happy, you got them when you moved in there, use them.
White people can help mostly by staying out of the way and letting this happen. Stop fucking over the tribal people who are trying to fix the problem white people created.
Here are some traditional and modern recipes for wild rice. It is better if you have manoomin, but in most places all you can get is cultivated "wild rice" which is nowhere near as good. (The strains and processing are different, with different results.)