HumCo GC856 T56
Humboldt Bay is a coastal lagoon, 14 mi. long and a maximum of 4 mi. wide. The bay has three distinct segments, South, Entrance, and Arcata Bays, each of which occupies the seaward end of one or more stream valleys cut into Plio- Pleistocene paralic sediments. The segments are linked by narrow tidal channels which are bounded by a barrier spit on the west and by high valley interfluves on the east. Climate of the area is rainy temperate . Tides are of the mixed type with a mean range of 6.5 ft.
Textural variations of the surface sediments in South and Arcata Bays correlate generally with bay-floor morphology. Bottoms of the inward branching tidal channels are covered by gravelly, shelly sand which becomes finer and more muddy with increasing distance from t he tidal inlet. Clayey silt predominates on the tidal flats, and highly organic silty clay or clayey peat occurs in the few remaining salt marshes. Tidal flat sediments are olive gray to black and thoroughly stirred by the rich bay infauna. The general pattern of decreasing grain size with increased elevation and distance away from the inlet is controlled by tidal currents. Exceptions to the general pattern and marked textural differences between tidal flat sediments in various of the bay segments result from: direct sediment discharge by entering streams, variable intensi ty of wave action, corrmercial oyster harvesting, and variations in flushing time. Dredged channels have larger components of both gravel and mud than their undredged counterparts.
Study of old maps and aerial photographs coupled with nearly ubiquitous evidence of marsh erosion indicate the bay is in approximate equilibrium, that is, filling at rates the order of 0.2-0.4 cm/yr commensurate with relative sea level rise. Most of the sediment comes indirectly from the Eel and Mad Rivers by way of littoral and tidal currents . Direct measurements show local accretion and erosion rates up to 4 cm/ yr and 11 cm/yr respectively. These fluctuate on a seasonal basis in response to alternating wind-wave patterns and reflect 'in-bay' relocation of materials.
Presence of extensive tidal flats and salt marsh imply former higher rates of accretion and bay infill. The recent change toward equilibrium conditions may relate to sediment removal by dredging and/or to a reduction in sed iment supply caused by northward shift of the Mad River course.