Kennebecasis -Grand Bay Sill
A view of the salt and fresh water exchange in the lower St. John River

John E. Hughes Clarke and John Winistok
Ocean Mapping Group, Dept. Geodesy and Geomatics Engineering
University of New Brunswick, CANADA
506-453-4568 (phone), 506-453-4943 (fax),

Over the summer and autumn of 2000, an oceanographic investigation of the exchange of fresh and salt water was conducted over the sill that separates the Kennebecasis fjord-estuary from the St. John River outflow and the tidal influx of salt water through the Reversing Falls. Four components of the survey were conducted:

  1. a bathymetric and sidescan map of the critical constraining shoal area was built.
  2.  200 kHz acoustic backscatter profiles of the water column were obtained over two complete tidal cycles
  3.  simultaneous 600 kHz ADCP current profiles were obtained of the same profiles
  4. remote, automated TS profiles were obtained using the Ocean Sensors OS500- APV
These surveys  built on the previous results of the May multibeam survey of the SW Kennebecasis performed as part of SE5083- Hydrographic Field Operations.

Area of Interest
The area of interest is the sill that separates Grand Bay from the Kennebecasis Basin. The aim of the surveys were to investigate the exchange and mixing  of salt and fresh water over that sill.

Oceanographic Framework of the Lower St. John River and the Kennebecasis Estuary
Salt water penetration up the St. John River extends to Oak Point. At this location, the river shoalohn River extends to Oak Point. At this location, the river shoals to less than 10m for several miles and thus there is no further opportunity for the salt wedge to penetrate. The St. John river drains central New Brunswick and Northern Maine with an average discharge of ~35,000 c.f.s. with significantly higher peaks discharges (240,000 c.f.s)  during the spring freshet. The fresh water outflow forms a stable 5-10m layer over the top of the salt water wedge.

The fresh water flow through the Kennebecasis, in contrast is minimal (the discharge of the Hampton River is a tiny fraction of the St. John). Nevertheless a similar stable 5-10m layer of fresh water overlies a salt plug that fills the remainder of the basin (with a maximum depth of 62m, although mean depths are ~30-40m).

Previous work...
The survey of the Reversing Falls

Water Column Instrumentation
combined surveys were done using a Knudsen 200 kHz narrow beam sounder together with an RDI 600 kHz Monitor acoustic doppler current profiler.

much more details on the ADCP surveys, including animation....

In order to ground truth and quantify the acoustic imagery, repetitive temperature and salinity profiles were taken using an autonomous, bottom mounted profiler. 

Bathymetric Surface Creation
To better establish the physical constraints imposed by the sill topography in this area, a new bathymetric survey was done, combining multibeam data collected in a survey from May 2000 with single beam data collected in June. A by-product of both these surveys was the mapping of the surficial backscatter strength in the area. The backscatter data can be used as an indicator of surficial sediment type.

EM3000 multibeam bathymetry

KEL 320BP single beam bathymetry

combined and interpolated bathymetric surface

EM3000 300 kHz backscatter image

KEL 200 kHz sidescan backscatter

Sill-top bathymetric and sidescan survey results
Two major erosional channels were discovered in the May multibeam survey. The June surveys was able to extrapolate the extent of these channels and establish that there were several, previously unreported, erosional corridors across the top of the sill. Because the sill top depth is close to the average level of the halocline in the area, these channels represent sites of enhanced exchange of the lower salt water layer.

Initial Conclusions
As notes in previous literature (New 1960, Trites, 1060) the sill acts to constrain the exchange of salt water between Grand Bay and the Kennebecasis. New results indicate that this exchange is actually focussed within shallow channels within which flow-intensification and enhanced turbulence have been imaged. A second "Reversing Falls" takes place twice a day (under periods of low river discharge) at the interface between the salt and fresh water.



Neu, H.A., 1960, Hydrographic survey of Saint John harbour, NB.: National Research Council of Canada, Mechanical Engineering Report MH-97.

Trites,R.W., 1960, An oceanographical and biological reconnaissance of Kennebecasis Bay and the Saint John River Estuary: Journal of Fisheries Research Board of Canda, v.17, p.377-408.

still under construction....

last modified  12th November 2000 by John E. Hughes Clarke. dy>