By lowering an electronic sensor package through the water using
a winch we can measure the changes in water properties with
depth. Temperature and salinity are basic parameters for
oceanography, and can be used to compute water density. Ocean
water is stratified - light water lies on top of heavy water -
and changes in density from place to place drive currents. When
heavy water appears on the continental shelf it can flow into
the Sound, replacing the existing deep water in a process called
"renewal".  Measuring the dissolved oxygen content helps in
identifying this process - newer water tends to be higher in
dissolved oxygen content than older water, because as water sits
in one place decomposition of organic debris sinking down uses
up oxygen.   The plots below show water properties at the
"Sarita Hole" Station, which is located in a 200m deep "hole" in
Trevor Channel, off the Sarita River. Water properties here are
representative of those over all of Trevor Channel and the lower
reaches of Alberni Inlet.