Juvenile salmon reside in estuaries for weeks or months as they grow and adapt to salt water before moving out to sea. Here is where salmon make a critical transformation from a freshwater to a saltwater fish, a process called smoltification. Changes in body chemistry, appearance, and behavior occur. When salmon return from sea as adults, they pause again in estuaries to adapt to freshwater, before heading upstream to spawn. By mapping the maximum extent of the tidal salinity intrusion up the Columbia River, we can learn more about the spatial distribution of salmon habitat, as well as how this habitat is changing from season to season and year to year.
Figure 1 shows the maximum intrusion for two very different years, near the time of the maximum annual flow, the spring freshet. Figure 2 shows a map of the maximum observed salinity intrusion distance, from October 2000. This research is also interesting in light of the recent debate over channel deepening in the lower Columbia River; this Army Corps of Engineers project is expected to increase the salt intrusion distance, thereby changing the spatial distribution of brackish water and freshwater habitats along this stretch of the river.
Figure 1: The Columbia River estuary and its north and south channels are shown in the top panel. The lower (south) channel is the deepwater shipping channel, and therefore the channel of maximum salt intrusion. Transects up this channel are highlighted in the two lower panels, which show a two year comparison of the neap tide estuarine salt intrusion at a similar tidal stage. Salt intrusion distance is maximal during the higher high tide of neap tides. River flow values are shown for Bonneville Dam outflow (150 miles upriver; appropriate time lag was applied).
Figure 2: Salinity contours (black) and current speed (during flood tide) when the maximum upriver salt intrusion of 49km was observed, October 6th, 2000. River flow for this period was 3400 m3s-1, though flow was changing rapidly during this period.