Chilmark Pond: A Successful Spring Cut Provides Hope for a Healthy Summer Pond Ecosystem

Chilmark Pond 2026 Spring Cut

Chilmark Pond (CHP) was successfully opened to the Atlantic Ocean on Thursday, March 10th, 2026, and presumably closed on April 12^th, 2026, resulting in 32 days of tidal flushing! This cut marked an important milestone following the completion of dredging efforts this winter. Using an excavator, the barrier beach is breached by the Chilmark Pond Association, creating a temporary channel that connects the Pond to the ocean.

Once the opening is made, Pond water begins rushing into the ocean, initiating a rapid drainage phase which lowers the Pond elevation and flushes out the water. Over the next day, this outflow naturally widens the channel, allowing even greater volumes of water to pass through (Photo 1).

Photo 1: Photo of CHP cut channel taken on March 13^th three days after the Pond was open to the ocean. Photo courtesy of Ollie Becker.

Following the drainage phase, which lasts about 24 hours, tidal exchange begins. Once the Pond becomes tidal, it exchanges water with the ocean, bringing in cool, clean saltwater and flushing nutrient‑rich Pond water out to sea. This regularly renewed tidal connection plays a vital role in maintaining water quality and supporting the overall health of Chilmark Pond.

Cuts Are an Effective and Efficient Management Tool

Managing Water Levels & Protecting ShorelinesPeriodic openings are a key management tool for the Chilmark Pond ecosystem. By lowering elevated water levels, cuts help prevent flooding of adjacent low-lying areas and reduce erosion of shorelines. Without intervention, prolonged high water can put both infrastructure and natural structures at risk.

Flushing Excess NutrientsPond openings also help improve water quality by flushing nutrient-rich water to the ocean and replacing pond water with nutrient-deficient seawater. Over time, nutrients like nitrogen can accumulate in the Pond and contribute to issues such as algal blooms and reduced oxygen levels. By periodically flushing the ecosystem, cuts act as a natural “release valve” helping to maintain a healthier and more resilient pond environment.

Increase Salinity Pond-Wide

The introduction of ocean water during a cut increases salinity throughout the lower Pond. Higher salinity levels can discourage excessive algal growth while supporting species that rely on brackish conditions.[ER1] 

Allow Exchange of Biological Organisms

Cuts allow for the exchange of marine and estuarine life between the Pond and the ocean. Fish, shellfish, and other organisms can move more freely during this period, supporting biodiversity and strengthening the overall ecosystem.

During the Summer, Cuts Help Lower Water Temperature

During the warmer months, openings can help lower pond water temperatures. The influx of cooler ocean water reduces thermal stress on aquatic life, increases dissolved oxygen, and reduces algal growth.

Important Note! Although these openings are critical, they are also dependent on natural conditions and cannot be relied on as the sole nutrient management tool. Water levels must rise sufficiently, typically through precipitation, before a cut is possible, meaning timing and frequency of cuts vary from year to year.

Drainage Phase, Tidal Phase, and Elevation

At the time of opening, Chilmark Pond’s elevation measured at 4.97 ft. relative to NAVD88 (Figure 1). The initial drainage following the cut was exceptionally strong, so much so that water levels dropped below the elevation logger, which is mounted just a few inches above the Pond bottom on a dock (Photo 2). As a result, precise measurements of the full initial drop are unavailable. However, hand measurements indicate that this opening caused the elevation of the Pond to drop more than 1 foot lower than has typically been the case over the last 3 years.

After this initial drainage phase, the Pond exhibited strong tidal signal. The tidal swings between high and low tide reached almost a full foot indicating effective and sustained water exchange between the ocean and the Pond!

Photo 2: Photo of the CHP elevation logger at the end of the dock out of the water.

Figure 1: Readout of Chilmark Pond elevation data relative to NAVD88 from March 10^th, 2026, to April 15th, 2026. The “bottoming out” or flatlining in the data is from the waterline becoming lower than the logger attached on the dock. The cut presumably closed on April 12^th, 2026.

Photo 3: The photo on the left was taken on January 8th, 2026 showing CHP’s water levels before the March 10^th, 2026 cut. The photo on the right was taken on March 13^th, 2026, showing the water levels after the initial drainage phase following the cut.

CHP System-Wide Impact

When CHP is cut, elevation changes are often seen in the lower Pond basin where the barrier beach is breached, however noticeable drainage to the upper and middle Pond basins is not always evident. However, with this current cut, the middle was noticeably lower in elevation 1 day after the cut had closed (Photo 4). This tells us that the drainage was strong enough to impact the whole Chilmark Pond system and not just the lower Pond. This could be promising for a healthier ecosystem in the upper ponds during the hot summer season when blooms are high. Stay tuned, we’ll be monitoring and comparing 2026 to previous seasons. This excellent and effective post-dredging cut combined with the spring launch of Sonic Buoy might be a game changer for the ecosystem.

Photo 4: Photo of CHP middle pond low water elevation taken on 4/13.

Salinity

Conductivity loggers are deployed in all Chilmark Pond basins (lower, middle and upper) year-round (Photo 5). These loggers give us consistent salinity levels throughout the year to better understand this intricate ecosystem and the impacts of the cuts.

For context, the salinity of ocean water is 32-36 ppt, while freshwater is 0 ppt; anything in between is considered brackish. Prior to the cut, salinity in Thumb Cove (in the lower basin) had dropped below 5 ppt. In late March (earlier in the cut’s lifespan), salinity in the Thumb Cove reached 30 ppt before levelling out closer to 25 ppt. Three days after the cut, Wednesday, April 15^th, the salinity in Thumb Cove was around 20 parts per thousand (ppt) (Figure 2). This would put salinity in the upper brackish range and indicates a strong flushing of the Pond.

Photo 5: Science team checking a conductivity logger (attached to a “bottom feeder”) in Chilmark Pond middle basin during the offseason.

Figure 2: Readout of Thumb Cove (located in Chilmark Pond lower basin) salinity data (ppt) from early March 2026 to mid April 2026 showing salinity levels before and after the Spring cut. NOTE: Since the Pond elevation dropped so low following this cut, the conductively logger in the lower basin of Chilmark Pond ran into the same issue as the elevation logger, bottoming out due to the sensor being out of the water. To correct this, all data points that were logged when the sensor was out of the water were removed.

Dredging; An Attributor to This Successful Cut

The accumulation of sand along the Pond side of the barrier beach can significantly shorten the duration of a cut, limiting water exchange and reducing the Pond’s overall flushing capacity. To address this issue, our dredge team worked for over two months this winter and removed almost 6,000 cubic yards of sand from the area inside of the cut and placed in back on the barrier beach (Photo 6).

Photo 6: Dredging in the Lower Chilmark Pond. Photo Courtesy of Ollie Becker.

This project focused on creating two channels near the typical cut location, removing built-up sand to improve drainage, and promoting more effective tidal exchange between the Pond and the ocean during cuts.

Shortly after the project’s completion, Chilmark Pond (CHP) was opened on January 23, 2026. However, that cut remained open for only three days. Unfavorable weather conditions and tidal patterns limited the cut duration, preventing a full evaluation of the dredging efforts.

The positive effects of dredging became evident soon after, as the March 2026 opening provided a more accurate assessment of the project’s success and is the longest cut CHP has had since an 81- day opening in the winter of 2018-2019. The unusually long duration and strong drainage during this cut highlight the effectiveness of the dredging work in improving water exchange and Pond openings.