 Sunlight is a critical resource for aquatic ecosystems. Light controls the growth of plants and algae that form the base of aquatic food webs. It degrades contaminants, and it can even affect animal behavior. But underwater light regimes within rivers are still a mystery. Large rivers are often deep, dark, and turbulent, which creates challenging conditions for measuring light and its effects on ecosystems. So we decided to flip our perspective and look at rivers through the eyes of the organisms that go with the flow. Enter the hydrosphere. It's a new robot-like instrument that passively moves through a river like a neutrally buoyant particle. It measures light and water quality at various depths in a water column. We deployed the hydrospheres in two rivers, the Upper Mississippi in Wisconsin and the Neuse River in North Carolina. And we found three compelling results. First, organisms that go with the flow are exposed to less light than models predict. Second, light exposure was flashy. A drifting organism may receive 60 to 99% of its daily light exposure over just a few minutes. And third, light exposure was three times more variable for drifting organisms than plants rooted to the river bottom. These findings could improve how scientists and managers predict algal and plant biomass in rivers and the spread of contaminants downstream. Looking at rivers through the eyes of organisms that go with the flow offers new insights into the underwater world of rivers.