Territorial problems such as the socio-ecological degradation of urban rivers represent a great challenge to achieving sustainability in cities. This issue demands collaborative efforts and the crossing of boundaries determined by actors that act from diverse spheres of knowledge and systems of practice. Based on an integrative territory notion and the boundary approach, the goal of this paper is to comprehend the boundary crossings that take place in multi-actor initiatives towards the resolution of this problem and what type of territorial transformation is produced as an outcome. Our analysis is built on participatory research on the Taquara Stream case, a degraded watercourse in a socio-ecologically vulnerable area, in southern Brazil. Our data analysis applied a visual chronological narrative and an interdisciplinary theoretical framework of analysis that combined concepts related to the territory (geography) and the boundary approach (education). We verified that local territorial issues functioned as boundary objects, fostering and facilitating dialogical interaction among involved actors, knowledge co-production, and collaborative practical actions that led to changes in the territory in terms of practices, comprehensions, and physical concrete transformations. We framed this study as one of territory-based learning meant to advance the understanding of territorial intervention processes towards urban sustainability.Sustainability 2020, 12, 3000 2 of 19 different interests, forms of knowledge, systems of practice, capacities for articulation, and influence in decision-making [7][8][9][10][11].Territory is understood here as an entity emerging from the interconnected dynamics between society and nature [12,13] and regarded as an important concept for critically reflecting on the socio-environmental processes enacted by initiatives towards urban river regeneration. The territory is formed by the interplay of diverse systems of practice, ideologies, and forces engendered by the social actors interacting and co-producing these evolving socionatural spaces [14]. It implies diverse modes of collaboration, forms of appropriation, and power relations [15,16]. From this standpoint, we argue that initiatives aiming at the transformation of the harsh reality engendered by the degradation of urban rivers must be comprehended not just in terms of the desired concrete biophysical changes they should achieve but as actions embedded in territories that deal with territorial issues and all the complexity conveyed by this notion.The search for solutions to such territorial problems implies collaborative efforts and the crossing of boundaries established by actors that act relatively independently from diverse spheres of knowledge and practice [9,11,[17][18][19]. Such boundary crossing can take place when a group composed of representatives of different social actors act in concert, interact and conduct dialogue, critically reflect together, and collectively learn [20,21] so as to co-create sustainability-oriented solutions...