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Bute Inlet

Physical Description

Length: 75 km
Mean Width: 3.7 km
Maximum Depth: 650 m
Major Basins: one main deep basin
Sill Depth: 370 m, just outside the inlet in Calm Channel

Map and Section

Runoff

There are two main rivers that discharge into Bute Inlet,  the Homathko and the Southgate, and both enter at the head of the inlet.  The Homathko,  the largest river, drains an interior watershed (~ 5720 km2) containing several permanent ice fields.  Trites (1955)  estimated the total mean annual freshwater flow into the inlet to be approximately  410 m3 s-1 of which  280 m3 s-1 is from the Homathko River.  Runoff to Bute Inlet is highly seasonal, peaking in July because of snow and ice melt from the interior watershed.  The lowest discharge occurs in the winter months because  most of the precipitation is stored as snow in the higher elevations of the watershed.

Deep Water Renewal Process

Bute Inlet is a deep inlet located amongst the narrow tidal passages at the northern end of the Strait of Georgia.   The main passage for the deep waters of Bute Inlet is to the south over the very deep (370 m) sill in Calm Channel and then through Sutil Channel to the Strait of Georgia .  The passages to the northern waters of Johnstone Strait and Queen Charlotte Strait are constricted and shallow.    Tabata (1957)  reported on the early oceanographic surveys of Bute Inlet, and later Lafond and Pickard (1975) described the deep water renewal process.   Using a series of observations from 22 cruises over a two year period from 1972 to 1974  Lafond and Pickard (1975) documented several inflow events.  The renewal of the deepest (>400 m) waters occurs in the 3rd and 4th quarter of the year.  The timing of the renewal of Bute Inlet deep waters is supported by the observed  fourth quarter maximum in salinity in Sutil Channel (Pickard 1975) and consistent with the annual deep water cycle in the Strait of Georgia. 

Time Series of Deep Water Properties

Temperature
  • almost linear increase in temperature from 1975 to 1999 of about 1 C
  • in the first half of the time series there are large (~0.5 C)  temperature fluctuations
  • warmest deep water temperatures observed in 1999
  • coldest deep water temperatures observed in 1957 

Salinity
  • no trend evident
  • lowest 400 m and 500 m salinities coincident with lowest temperatures in 1957

Dissolved Oxygen
  • no trend evident
  • large amplitude (~1 ml/l) variations in first half of time series
  • lowest dissolved oxygen values recorded in 1998 and 1999