In regions of large horizontal density gradient, tidal straining acts to pr
oduce a periodic component of stratification that interacts with turbulent
mixing to control water column structure and flow. A 25-h series of measure
ments of the rate of dissipation of turbulent kinetic energy (epsilon) in t
he Liverpool Bay region of freshwater influence (ROFI) have revealed the fo
rm of this interaction and indicate substantial differences from regions wh
ere horizontal gradients are weak. In the ROFI system there is a pronounced
difference between flood and ebb regimes. During the ebb the water column
stratifies and strong dissipation is confined to the lower half of the wate
r column. By contrast, during the flood, stratification is eroded with comp
lete vertical mixing occurring at high water and high values of dissipation
(3 mW m(-3)) extending throughout the water column. The cycle of dissipati
on is therefore predominantly semidiurnal in the upper layers whereas, near
the bottom boundary, the principal variation is at the M-4 frequency as ob
served in regions of horizontal uniformity. Toward the end of the flood pha
se of the cycle, tidal straining produces instabilities in the water column
that release additional energy for convective mixing. Confirmation of incr
eased vertical motions throughout the water column during the late flood an
d at high water is provided by measurements of vertical velocity and the er
ror velocity from a bottom-mounted acoustic Doppler current profiler.