185 
any certainty upon the possible effects of variation of the concentra- 
tions of suspended sediments. 
THE ROLE OF MAN 
I have discussed, where sufficient evidence exists, the possible 
roles of sediments and sedimentary processes with regard to primary 
production. This final section deals with the ways in which man 
may influence the productivity of wet lands through actions which 
influence the sedimentary regime. Man’s influence may take two 
basic forms: (1) He may vary the amount of sediment in the estuary 
(or its distribution), and (2) he may alter the nature of the material 
entering the wetlands system. The Jatter would include sewage and 
industria] waste problems, problems involving insecticides and other 
particulate toxins, and so forth. These topics are receiving considerable 
attention, and will not be discussed here. 
Man may alter the amount of sediment in the wet lands by a wide 
variety of actions, which may be divided into those actions which 
increase the sediment supply, and those which decrease it. Among 
the former, deforestation, agricultural practices, dredging, filling, 
and in some cases flood control, are among the most important. Of 
those actions which decrease sediment supply, flood control is the 
most important, involving the construction of dams, canals, spillways, 
and levees. 
It is difficult to estimate the degree to which man can increase the 
amount of sediment reaching the estuary with any precision, because 
there are so few estuaries which have relatively undisturbed watersheds 
to use as references. For example, virtually all the virgin forest in the 
Chesapeake Bay watershed had been utilized before 1900, and erosion, 
siltation, and sedimentation which accompanied the deforestation 
increased throughout the 1800’s. The magnitude of the cumulative 
effect is suggested by the estimate that half of the former upper 
estuarine areas for fish spawning and shellfishery areas for oysters have 
been destroyed or shifted downstream in Chesapeake Bay." 
There is no doubt that deforestation and poor agricultural practices 
imerease wetlands sedimentation. What is more difficult to establish is 
the effect, in the long run, of such increases. It is also clear that if it 
can be shown that increases in sedimentation are detrimental to 
primary production, then a case can be made for measures such as 
effective soil conservation, small watershed dams, and larger resevoirs, 
all of which are effective in retaining silt. 
Dredging and filling are now common wetland “reclamation’’ 
techniques. Due to the possible deleterious effects on the commercial 
shellfishery, the effects of dredging operations has been investi- 
gated 41516 but there is as yet no study relating dredging operations 
to primary production. Generally, the effects of channel dredging are 
relatively local, but silt deposition has been reported as thick as 27 
centimeters and as far as 0.5 mile from the dredging site.’® In addition 
to the stirring up of sediment by dredging, wave action resuspends 
sediment from the spoil bank for long periods after the operation has 
ceased, creating a possible long-term increase in suspended sediment 
concentration. 
Footnotes at end of article. 
