By Bill Romey
Mill Pond occupies three small basins at the southeast edge of Nauset Harbor. It’s about a half a mile long and connects to the rest of Nauset Harbor by a long, narrow channel through Roberts Cove. On an air photo its outline looks a little like a steamer clam with its neck extended. Farmer Oliver Doane made a stone dam across the narrow opening to the pond in the late 18th century and built a gristmill there to grind his grain.
The dam remains there, mostly submerged, with two narrow gaps that allow boats to pass at low tide. At high tide boats pass easily over its top. A deep pool on the Mill-Pond side of the dam, where the incoming tide cascades over the dam, makes a good place for SCUBA diving, and it serves as home to all sorts of fish, eels, shellfish, crabs, lobsters, starfish, urchins, hermit crabs, squid, and an occasional seal. Terns, gulls, geese, swans, ducks, egrets, kingfishers, herons, crows, foxes, raccoons, skunks, possums, otters, and other small mammals hang around to harvest the pond’s bounty. Ospreys, harriers, red-tailed hawks, buzzards and occasionally bald eagles sail overhead.
We humans who live on the pond like to know when high and low tides are likely to happen on a given day. Knowing this is important to fishermen, boaters, and divers. For the fishermen, high water is a good time to have a line in the water since the fish are up off the bottom, feeding on goodies the tide brings in from the open ocean beyond Nauset Inlet. Boaters caught inside the pond at very low tides have trouble getting back out of the pond when there is only a foot of water over the two openings in the dam. Sailboats get hung up on the reef just inside the mill race at low tide. The inrushing tide tumbles divers mercilessly and can overturns boats. We nearly lost a prospective son-in-law when he got caught in the swirl of the tidal rapids and his canoe turned over.
For a long time I was victim of the belief, reinforced by the published tide charts, that high and low tides follow each other at handy six-and-a-quarter-hour intervals. Tide clocks further propagate the myth. Beginning in 1979 when we first stayed on the Cape for a full year I had noticed that things seemed out of synch in Mill Pond, but it wasn’t until I started keeping charts of my own that the truth came out. Daily measurements soon revealed that the flood tide was coming in more rapidly than “scheduled” and the tide was taking a longer time to ebb than expected. The time for the tide to come in was taking only 4 hours with the ebbing lasting 8 hours.
I took a series of photos and measurements in front of our house every 15 minutes during the day for a full 13-hour period. Planting sticks at the water’s edge to show the water level, I started taking pictures at full high tide and then took another picture every 15 minutes. At the end of six hours I expected the water to be at its lowest level. But it kept going down for more than two hours. Once it hit its lowest mark, the tide level literally leaped up from its lowest to its highest point in only four hours.
I then took a similar sequence of photos at the mill race. There, when the tide came in over the dam, a “bulge” of water up to a foot and a half high, a tidal bore, moved in against the dam, creating turbulent rapids through the narrow opening for an hour or so. Once the level of the water reached the top of the dam the current flowed in more gently. When the tide was ebbing, the current, although still strong enough to turn a mill wheel, never made rapids like the ones caused by the flooding tide,
In 1994 we moved full time to the house on Mill Pond. Then I was in a position to watch what the tides were doing regularly. Up until 2012 there were still substantial rapids. Then several people on the pond mentioned that they were no longer hearing the loud noise of the rapids. I had also noticed that the tide didn’t seem to be reaching as low a level on the dam as in previous years, and boats seemed to be passing over the dam more easily. In late August, 2015 I took another sequence of photos of the tide level at the mill race. There no longer seemed to be any rapids. The tidal bulge in Roberts Cove that had preceded the rapids for many years was now much lower than previously. The incoming tide water still flowed noticeably, but the current wasn’t sufficient to create real rapids. The interval between high and low tide remained the same however, four hours to flood, and eight hours to ebb.
High tide in Mill Pond comes one hour after the scheduled time for high tide at Nauset Inlet (using the Goose Hummock chart). Low tide at Mill Pond comes eight hours later (nine hours after high tide at the outer beach). Thus the tide continues to ebb out of Mill Pond for three hours during which the new tide is already rising on the outer beach. Water continues to flow out of Nauset Harbor while new ocean water flows in through the inlet. At the inlet, the sill made by the outer beach delays the entry of new ocean water into the harbor for a while, too. That explains the unexpected directions of currents throughout the harbor.
Mill Pond’s tide schedule is not unique. Eastham’s Salt Pond has a similar asymmetrical tide pattern, four hours in and eight hours out, as a result of its long, narrow entry channel, and Town Cove doesn’t fit the intervals predicted by the tide charts either, but the length of ebb and flow is more nearly equal. As you plan outings, be prepared for the crazy tides flowing into and out of the harbor’s many small basins. Remember that the flood tide currents in some areas are still strong enough to turn a small boat upside down, too. It’s always interesting to watch inexperienced kayakers going sideways through the race because they did not understand the tides. If you are going to Mill Pond, remember that the tide charts in the papers aren’t applicable. The tides are asymmetrical. The charts are fine for the outer beach or bay-side beaches. I’m always sorry to see people arriving at Mill Pond and Roberts Cove, hoping to take some shellfish, only to find that low tide is still several hours away. Perhaps, however, that leaves more shellfish for me!
Why are there asymmetrical tides? Asymmetrical tides such as the ones in Mill Pond result from the overall geography of Nauset Harbor, with narrow connecting channels and inlets restricting the flow of the water into some basins. Such tides are common in estuaries around the world
Possible reasons for the reduction of the rapids into Mill Pond include the following: 1. Sea level is rising slightly, bit by bit, and the higher level raises the lowest tides enough to pass over the dam with less impediment. Low tide is not as low as it used to be. This is a result of global warming. 2. Nauset Harbor is slowly filling with more sand washing in from the inlet, disrupting the timing and level of tides in the inner basins. 3. The height of the dam has been reduced over recent years due to human activities and natural deterioration. All of these factors probably apply here. All are testable hypotheses.
-Bill Romey, lives in East Orleans and is a retired professor of Geology and Geography from St. Lawrence University.