The need to reconsider human-environment interactions is ever-pressing in light of increasing anthropogenic risks to waterways. Insights into how people connect with rivers and what matters to them as they experience rivers may enable us to promote river ecosystems' health better. In addition, impressions from experiences of outdoor environments documented and shared on social media may help us understand how environments like rivers matter to people. Materials captured in photos and posted on social media offer an alternative 'digital' record of visitor impressions of river experiences, revealing how visitors interpret and interact with rivers. This study focuses on materials representing relations between people and things and how things act on users and audiences of social media. Our analysis is based on Instagram posts about the Warta River. Through an AI-based approach, we uncover material aspects of the Warta River experiences in social media visual posts, allowing for the generation of knowledge about how visitors relate to the waterway.