In the late 1910s the notch area was slightly disturbed when New York City began building the Shandaken Tunnel, allowing water from Schoharie Reservoir to flow underground to the Esopus near Shandaken, where it would eventually reach Ashokan Reservoir. It followed the road through the notch, and its air vents and a cleared right-of-way can be seen where it crosses Route 42 north of the notch. Under the slopes of Balsam Mountain just east of Deep Notch, it reaches its greatest depth below the surface, .