Dunes are the most common bedform in sand-bedded rivers. Small, high-angle dunes (HADs) dominate in shallow (<2.5 m) flows and have lee sides with steep downstream-facing slipfaces (>24°) and reversed upslope-oriented flow in the separation vortex. In contrast, large, deep (>2.5 m) rivers have low-angle dunes (LADs) with gentle lee-side slipfaces (<24°; often <10°), little or no flow separation, and more-pronounced downslope currents. Avalanches driven primarily by particle interaction maintain slipfaces of HADs. On LADs, excess pore pressures that occur during the failure of thick, loosely packed deposits can produce liquefied avalanches that flow and stop on gentle gradients. As lee-side angles decrease over LADs, downslope currents increase in strength, accelerating avalanches, transporting bedload, and creating smaller, superimposed dunes and uniform concave slopes on the lee side. Nearly a century of research on dunes in shallow laboratory flows has assumed that dune morphodynamics are scale invariant, which is not true.