Water age in the Columbia River estuary

The concept of water age is applied to the Columbia River estuary to investigate water renewing time scales. Water age tracers were implemented in a three-dimensional circulation model. The model was run for a nine month period in 2012, covering both high and low flow conditions. In the lower estuary renewing water age ranges from roughly 20 h during high flow season (typically April-June) to 70 h during lowest river discharge (typically September-October). The age of riverine water is strongly dependent on river discharge. Dense oceanic waters, in contrast, are always relatively young in the estuary (roughly 20 h) although their age does vary with tidal range and river discharge. Compared to the main channels, water age tends to be larger in the lateral bays throughout the simulation period; this is especially true under low flow and neap tides conditions when water age can exceed 120 h in the bays. During low flow conditions a strong lateral circulation pattern emerges and leads to higher water age near Grays Bay. The maximal water age in the main channels is associated with mixed water mass (around 6-12 psu) located in front and above the salt wedge. The circulation model results are used to derive simple regression models that can be used to predict renewing water time scales without the need of a circulation model.