A small stone such as this sitting in the middle of a prairie stream becomes a microhabitat. With a dry surface above the water it may serve as a resting site for flying insects. When it is underwater, it disturbs the flow and may help add oxygen to the water downstream. At this time of year, its dark surface heats up melting the surrounding ice and increasing the water flow beneath the ice, which may be important for over-wintering insect larvae. It all seems very insignificant unless you are an active member of the stream’s community of living critters.