Processes controlling stratification on the northern Skagit Bay tidal flats

Abstract

Estuarine systems range from highly stratified to well mixed, often passing through several states during a single tidal cycle. Here, the processes driving temporal and spatial variations of stratification are investigated on the broad, shallow, periodically inundated tidal flats (shoals) between the north and south forks of the Skagit River. Over a two-week period, observation-based estimates of the straining, advection, and mixing are in balance with the temporal changes in the stratification-induced potential energy anomaly at a mid-flat location. The water on the tidal flats does not completely de-stratify during the strong ebb, and thus the initial tongue of water crossing the flats on strong floods often is partly stratified, but becomes increasingly well mixed as the flood progresses. For nearly semi-diurnal tides, the maximum stratification occurs during mid-ebb tide. Although cross- and alongshore flows have similar magnitudes, the changes in stratification during semi-diurnal tides result primarily from cross-shore processes, similar to observations of narrow, strongly-forced salt-wedge estuaries. Stratification is stronger during tides with a large diurnal inequality (the elevations of the two daily low tides differ by more than 33% of the tidal range) than during nearly semi-diurnal tides (with low tide elevations that differ by less than 25% of the tidal range), and is a maximum in the middle of the weak flood that follows the small low. In contrast to prior observations in narrow estuaries, alongshore (roughly parallel to isopycnals) advection of stratified water is significant and contributes to the increased stratification during these tides. Furthermore, during the small low and weak flood, the strongly sheared (density-driven) flows, which are offshore-directed at the surface and onshore-directed at the bed, also contribute to the high levels of stratification. Temporal changes in stratification are similar across and along the flats. However, maximum stratification increases offshore and alongshore towards the north fork (the distributary channel closest to the instruments).