Massive Tidal Flows

River traffic. Piscataqua River currents restrict travel by tankers and other large vessels to slack tide, roughly four hours a day. See chart below for slack tide at the entrance to Portsmouth Harbor. The entrance is nearly three miles from the location of the PortsmouthNH.com's HarborCam. Slack tide at the Harbor Cam is roughly a half hour later. A schedule of shipping in the Piscataqua is not available. Currents. The swift currents of the Piscataqua River make Portsmouth Harbor one of the fastest flowing tidal waterways in the Continental United States (NOAA list of 50 fastest in North America). The ice-free deep-water channel has been used for shipping since the 1600s.