Increasing complexity of riverine management is driving the need for more participatory approaches in which a wide range of stakeholders inform management actions. The foundational conditions of successful stakeholder engagement in participatory riverine management include the stakeholders’ familiarity with, trustworthiness of, shared vision with, and perceived competence of agencies involved in riverine management. We employ an embedded experimental mixed-methods approach to gain insight into two key stakeholder group’s (agricultural producers and non-farm households) perceptions of foundational conditions of successful stakeholder engagement in participatory riverine management in watersheds of two geographically and regulatorily disparate states. Overall, ratings for familiarity were often contrary to ratings of other three conditions. Contrary to previous research our findings suggest that familiarity does not necessarily lead to stakeholder perceived trustworthiness, shared vision, and competence of those involved in riverine management. These findings are important for participatory processes; for example, trust and shared vision without familiarity may inhibit the active involvement of diverse stakeholders in participatory processes that is needed for improving management of complex riverine systems. Our findings suggest agencies and groups involved in riverine management prioritize developing stronger relationships with agricultural producers to improve on these foundational conditions. Agencies and groups involved in riverine management rated low on familiarity should consider campaigns to increase stakeholders’ familiarity with them to leverage the generally high ratings on the other three foundational conditions. Future research should explore these contrary findings and their relationships to active involvement in participatory riverine management.