||Region in an estuary where turbidity is high, due to concentration of particulates
in the water column. This occurs as a result of the increasing ionic strength of the
water from the introduction of salt in the downstream direction (generally at around
areas with a salinity of 5), which causes particle aggregation. This process is enhanced
by the mixing regime particular to this upper estuary region, in which countercurrent
flows from the salt wedge are entrained into the seaward-flowing, upper-water layer.
As particles settle out of the upper layer, they are carried upstream in the lower
layer and so circulate within a zone of maximum turbidity.