It seems a bit early in the year to have such low water in the river. I'm not concerned and I presume that this is simply the variability of the seasons. In my limited experience, August and September are usually months of low water …then tropical storms and hurricanes come and fill things up again.
What I am concerned about is the rapid rise and fall of the river's water level whenever we have a significant rain. It's probably due to increases in all manner of human development in the watershed. Also of concern is an increase in algae growth. This, once again is due almost exclusively to landowners relentlessly fertilizing grassy lawns and old septic systems.
Nevertheless, the low water always comes at a time when mallards are fully fledged and feeding on mats of algae and other slimy plant growths on the now-exposed face of the South Natick dam. It's fun to watch their antics as they clamor up and down like nimble feathered alpinists.