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The Bering‐Bagley Glacier System (BBGS), Alaska, Earth's largest temperate surging glacier, surged in 2008–2013. We use numerical modeling and satellite observations to investigate how surging in a large and complex glacier system differs from surging in smaller glaciers for which our current understanding of the surge phenomenon is based. With numerical simulations of a long quiescent phase and a short surge phase in the BBGS, we show that surging is more spatiotemporally complex in larger glaciers with multiple reservoir areas forming during quiescence which interact in a cascading manner when ice accelerates during the surge phase. For each phase, we analyze the simulated elevation‐change and ice‐velocity pattern, infer information on the evolving basal drainage system through hydropotential analysis, and supplement these findings with observational data such as CryoSat‐2 digital elevation maps. During the quiescent simulation, water drainage paths become increasingly lateral and hydropotential wells form indicating an expanding storage capacity of subglacial water. These results are attributed to local bedrock topography characterized by large subglacial ridges that dam the down‐glacier flow of ice and water. In the surge simulation, we model surge evolution through Bering Glacier's trunk by imposing a basal friction representation that mimics a propagating surge wave. As the surge progresses, drainage efficiency further degrades in the active surging‐zone from its already inefficient, end‐of‐quiescence state. Results from this study improve our knowledge of surging in large and complex systems which generalizes to glacial accelerations observed in outlet glaciers of Greenland, thus reducing uncertainty in modeling sea‐level rise. Plain Language Summary: The Bering‐Bagley Glacier System (BBGS), Alaska, Earth's largest temperate surging glacier, recently surged in 2008–2013. A surge glacier cycles between a long period of normal flow and a short period of accelerated flow where large‐scale deformations, such as crevasses, occur. This paper focuses on investigating a surge in a large and complex system rather than a small glacier where most studies on surges have been conducted. We use a numerical model to simulate glacier evolution for both the quiescent phase and the initial surge phase of the BBGS. For each phase, we analyze the simulated elevation‐change and ice‐velocity, and infer information on the evolving hydrologic drainage system. During the quiescent phase, ice‐mass builds up at locations consistent with those observed and water drainage paths become longer with expanding capacity to store subglacial water. These results are attributed to local bedrock topography characterized by large subglacial ridges that act to dam the down‐glacier flow of ice and water. In the surge simulation, we model surge evolution through Bering Glacier by implementing a new friction representation that mimics a propagating wave. As the surge progresses through the glacier, drainage efficiency further degrades in the areas of fast‐moving ice. Key Points: Using a full‐Stokes approach informed by satellite observations, a quiescent and surge phase of the Bering‐Bagley Glacier System (BBGS) are simulatedLocal bed topography controls the formation of several reservoir areas, which lengthen down‐glacier drainage paths during quiescenceA friction representation for the surge phase is implemented based on observed properties of kinematic surge waves in the BBGS [ABSTRACT FROM AUTHOR]