You aren't accounting for the fact that the marsh, on average, is shallower than the lake. I have taken measurements of gulf strength water (35 ppt) behind the weirs, but the salinity on the lake side was around 24 ppt.
Less water, higher salt content. Include evaporation in that, and the salt content in the marsh is higher than that of the lake. Also, during low water events, soil salinity can become hypersaline, and there are very few plants on the earth that can survive those conditions.
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