Trees play a critical role in the morphological, hydrological and ecological dynamics of rivers. Their root systems stabilise sediments, are key to regeneration after flood disturbance, and also create small scale morphological diversity upon exposure. However, very little is known about subterranean tree structures and controls on their distribution and form in the riparian setting. This book presents novel investigations from a dynamic and near-natural river dominated by black poplar, interpreting the results for wider understanding and management of river systems from individual tree to landscape scales. Following a comprehensive introduction, the three main chapters address a) root distributions within bank profiles, b) controlling environmental influences on these distributions, and c) whole root systems and buried stem structures. Wider implications for management, conservation and restoration are explored in the final chapter, as well as additional questions which emerge and preliminary results from supplementary further studies.