Fish biodiversity in rivers is threatened by continual habitat loss. Evidence to support management and restoration of rivers requires information about the specific habitat requirements for entire fish communities. The complexity of this topic, combined with additional challenges of multiple stressors, data limitation, and natural dynamics of rivers, facilitates novel model development for both applied and theoretical arenas. Central to this area of study is the importance of spatial scale. Spatial scale is the context on which measurements, models, management, and policy have the potential to synchronize in an effective manner. The forefront and progress of river restoration depends on the provisioning of timely evidence for decision-making that comes directly from models. 

This thesis explores and develops a spatial scale approach to model fish communities and habitat in impacted rivers. I present a comprehensive vision of how the spatial scale approach can be implemented across spatial scales. I then introduce new models at each individual scale: micro-scale, meso-scale, macro-scale, riverscape-scale, and beyond-riverscape-scale. For each scale, I studied a different river or system of rivers from around the world. The variety of modeling techniques I used highlighted new opportunities to extend models for methods in river restoration such as flow regime management, fish passage construction, dam removal, non-native fish regulation, and plastic pollution reduction. Advancements for future models and limitations are discussed in the context of spatial scale and interdisciplinarity. 

Fish habitat in rivers is under threat from severe anthropogenic impacts. Pollution, dams, water use, hydropeaking, and invasive species are now common in rivers around the world and their combined effects on riverine fish habitat have reduced freshwater fish biodiversity at an unprecedented rate. River managers rely on models to inform management and restoration in order to improve fish habitat. Historically, these models have prioritized individual fish species but managing fish biodiversity requires understanding of resources, risks, and conditions for entire fish communities. These components that make up the foundation of fish habitat have inherent scale dependencies. Thus, a more holistic approach to modeling will need greater attention to spatial scale, ranging from small streams up to entire watersheds. To move beyond current limitations, models must be capable to operate at a variety of spatial scales and address the most pressing issues in rivers. This thesis investigates fish habitat ecology in impacted rivers using a novel spatial scale approach. The approach improves the model development process for fish habitat in rivers. Using the approach, multiple models were developed that address specific impacts on fish communities in rivers around the world.