Sedimentation and sediment accretion in Michigan coastal wetlands (U.S.A.),

Abstract

Sediment transport and accretion rates were studied for three different types of coastal wetlands of Lake Michigan, U.S.A. A forested intradunal wetland (Cranberry Bog No. 1) near Stevensville, Michigan, displayed sediments in the form of detritus, and exported them. Very little sedimentary material entered by streamflow. These features of bottom-land wetlands were confirmed by radioisotope studies on cores, which showed low accretion rates of 210Pb. The mass fluxes were found to range from 0.01 to 0.2 g cm-2 yr.-1. The equivalent linear growths are 0.5-9.0 mm yr.-1. The suspended material from this wetland entered a filling shrub--bog--lake complex known as Cranberry Bog No. 2. A considerable fraction of the incoming sediments were retained in this intradunal basin wetland. This was confirmed via radioisotope studies, with cores showing higher accretion rates than in the bottom-land wetland. The river-delta wetland at Pentwater, Michigan, had little sediment trapping ability. Large amounts of water and suspended materials entered this riverine wetland, but no annual average removal was found. Radioisotope studies of cores at selected locations within this system showed little or no accretion in stream channels, and normal accumulation of organic material in non-channel areas. No net annual effect on water quality of the streams as they discharged to Lake Michigan was found. Storm events caused no notable import or export of suspended material. Presumably the sand substrate in the vicinity of all three wetlands caused runoff events to be mediated by subsurface flow through this porous soil. These three sand-based coastal wetlands appear to have little net effect on suspended solids.The 210Pb method proved more valuable than 137Cs because of the small depth of occurrence of the latter isotope. Interpretation of isotope profiles required information on acid-fraction-soluble profiles. Results are expressed in terms of mass units, since compaction of cores distorts the linear accretion rate. Delayed transfers of radionuclides appear to have occurred, perhaps as a result of integration of atmospheric inputs by the watersheds. The total activities of 210Pb and 137Cs are well correlated, which indicates particle trapping efficiency as the primary factor for affecting accumulation of both isotopes.