Tidal flat sediments - texture, Willapa Bay, Washington

Abstract

The purpose of this study was to texturally characterize the sediments of the mid-estuary tidal flats between Goose Point and Pickernell Creek in Willapa Bay, Washington and to compare these flats with other flats in the estuary. The mean grain size of sediments on tidal flats in the mid-estuary are fine sand (2-3 V). Sand shoals at the Bay entrance and erosion of cliffs between Goose Point and Pickernell Creek are the source of the sand. The subordinate mud fraction is supplied by rivers and creeks; little mud is deposited on the tidal flats except for those flats immediately adjacent to the rivers. Gravel is a minor component and is concentrated in sediments near the base of the cliffs high on the tidal flats. Variability in texture within the study area is most pronounced near the Palix River and Pickernell Creek. The average characteristics of mid-estuary tidal flat sediments are best described as poorly sorted, fine-grained sand with strongly-fine skewed leptokurtic distributions. Distribution patterns of textural parameters show consistent trends of hydraulic energy. Locally, textural patterns reflect (1) the coarsing effects of a flood current gyre between Goose Point and Sandy Point, and (2) relatively narrow flats (i.e., greater slopes) have coarser sediments proportionately higher upslope than wider flats. The following comparisons can be made between tide flats in Willapa Bay. Mean grain size varies systematically up-estuary permitting the estuary to be artificially divided into 3 parts: outer estuary = 2-3 Ø ; mid-estuary = 3-4 Ø ; and inner estuary = greater than 4 Ø . Outer estuarine tide flats extend deep into the estuary, particularly to the south. The transition from outer to inner estuarine tide flats based on textural differences is a rapid change (i.e., occurs over a short distance).