For 108 years now the dike has blocked tides and seawater from the 1100-acre estuary upstream. Nevertheless, most of the organic matter that accumulated within this expansive wetland over thousands of years remains.
Meanwhile, oxygen-rich Cape Cod Bay water, which would normally flood the river and creeks on every high tide, is greatly restricted by the dike at Chequesset Neck Road.
In summer, decomposition of this organic matter puts a serious strain on the river's dissolved oxygen budget. Indeed, during periods of low light for photosynthetic oxygen production, i.e. nighttime or cloudy weather, the river can be poised for complete oxygen depletion; the latter has been observed since at least the mid-1980s, accompanied by fish kills, in 1985 wiping out the entire juvenile river herring run when they attempted to journey from the kettle ponds to the sea. This has likely been going on for the past 100-plus years, severely damaging aquatic habitat throughout the flood plain, and perhaps explaining the apparent abundance of nuisance mosquitoes. Mosquito larvae and pupae don't require dissolved oxygen (they breathe air) and thrive in the absence of predatory estuarine fish.
The graph below shows dissolved oxygen trends from last summer (2016), collected by the US Geological Survey. The yellow box encompasses the concentration range that is chronically stressful to fish and other aquatic animals that "breathe water"; the red box marks the range that is acutely lethal to aquatic life. Oxygen varies with tides: the little bit of seawater that enters the river at high tide boosts dissolved oxygen, but during every low tide oxygen plummets as the water column becomes dominated by oxygen-depleted river water.
The second graph shows very recent data. On nearly every low tide in summer the river water becomes highly stressful to fish and other aquatic animals. (Imagine being in a room without air.) The problem will continue, quietly and lethally, until the river is re-connected with the marine environment.