Hydrography is the description of physical features of the ocean, and is the starting point for any analysis of water circulation and the understanding of life within the ocean.

Standard hydrographic properties in the ocean include water temperature, salinity, the dissolved oxygen content,  total chlorophyll (as a proxy for the total amount of microscopic plant life, or phytoplankton, that grows in the upper 30m or so of the water column where light can penetrate), and water clarity, which affects the depth to which light penetrates.

Most of these properties, from the surface down to 150m, are measured in the CitSci program by an electronic probe lowered from a boat. Water clarity is measured be separately estimating the Secchi depth.

The largest changes in the Strait of Georgia are seasonal, but over longer periods, there are noticeable changes from year to year as well in response to larger climatic variations such as El Nino cycles. The seasonal cycle will provide some background for the changes from year to year.

There are also regional differences. In order to show these, we have divided the Strait into 11 areas, and also provide averages in each of those areas.