Abstract Orogenic gold systems are open, flow-controlled thermodynamic systems and generally occur in mid- to upper crustal environments where there is strong coupling between fluid flow and dilatant plastic deformation. This paper considers the principles involved in such coupling, with an emphasis on the elastic and plastic volume changes and their influence on the fluid, mechanical and thermodynamic pressures. Some misconceptions regarding the magnitudes of these three distinctly different pressures and their influences on fluid flow and chemical equilibrium are addressed, with examples at both the tens of metres scale and the crustal scale. We show that the mean stress is less than twice the lithostatic stress for Mohr–Coulomb materials with cohesion and the thermodynamic pressure only has meaning under isentropic conditions and hence is less than many previously published estimates based on high mean stresses. At the crustal scale, we also include the role of critical behaviour in influencing the geometry and magnitudes of fluid pressure gradients and fluid flow velocities in open, flow-controlled systems.

Abstract For safety and environmental reasons, removal of aging dams is an increasingly common practice, but it also can lead to channel incision, bank erosion, and increased sediment loads downstream. The morphological and sedimentological effects of dam removal are not well understood, and few studies have tracked a reservoir for more than a year or two after dam breaching. Breaching and removal of obsolete milldams over the last century have caused widespread channel entrenchment and stream bank erosion in the Mid-Atlantic region, even along un-urbanized, forested stream reaches. We document here that rates of stream bank erosion in breached millponds remain relatively high for at least several decades after dam breaching. Cohesive, fine-grained banks remain near vertical and retreat laterally across the coarse-grained pre- reservoir substrate, leading to an increased channel width-to-depth ratio for high-stage flow in the stream corridor with time. Bank erosion rates in breached reservoirs decelerate with time, similar to recent observations of sediment flushing after the Marmot Dam removal in Oregon. Whereas mass movement plays an important role in bank failure, particularly immediately after dam breaching, we find that freeze-thaw processes play a major role in bank retreat during winter months for decades after dam removal. The implication of these findings is that this newly recognized source of sediment stored behind breached historic dams is sufficient to account for much of the high loads of fine-grained sediment carried in suspension in Mid-Atlantic Piedmont streams and contributed to the Chesapeake Bay.