It has been hypothesized that velocity reversals provide a mechanism for maintaining pool-riffle morphology in gravel-bed rivers-an important habitat for salmonids, which are at risk in many places worldwide and that are the focus of extensive environmental legislation in Europe and North America. However, the occurrence of velocity reversals has been controversial for over 3 decades. We present a simple one-dimensional criterion that unifies and explains previous disparate findings regarding the occurrence of velocity reversals. Results show that reversal depends critically on the ratio of riffle-to-pool width, residual pool depth ͑difference between pool and riffle elevations͒, and on the depth of flow over the riffle, suggesting that land management activities which alter channel form or divert water from the channel can have negative impacts on the sustainability of pool-riffle habitat in gravel-bed rivers.

The basis for and field trials of two leak detection algorithms by transient flow simulations are presented. Data noise in pressure and flow measurements are considered. Noise is found to limit leak detectability. The ability of the two algorithms to reliably detect small to moderate size leaks under steady and transient flow conditions is demonstrated.

We explored the potential and limitations for applying an acoustic camera as the imaging instrument of particle tracking velocimetry. The strength of the acoustic camera is its usability in low-visibility environments where conventional optical cameras are ineffective, while its applicability is limited by lower temporal and spatial resolutions. We conducted a series of experiments in which acoustic and optical cameras were used to simultaneously image the rotational motion of tracer particles, allowing for a comparison of the acoustic- and optical-based velocities. The results reveal that the greater fluctuations associated with the acoustic-based velocities are primarily attributed to the lower temporal resolution. The positive and negative biases induced by the lower spatial resolution are balanced, with the positive ones greater in magnitude but the negative ones greater in quantity. These biases reduce with the increase in the mean particle velocity and approach minimum as the mean velocity exceeds the threshold value that can be sensed by the acoustic camera.