Landslides at Delaware Water Gap: How One Park Learned Its True Risk

Park staff Jessica Dixon, Shary Kranch and Kelsey Taylor inspect a rockfall after a heavy rainstorm. NPS / Jonathan Malzone

At Delaware Water Gap National Recreation Area, slope failures — rockfalls, slumping stream banks, landslides — were rare in anyone's memory. Then, in spring 2022, a series of intense rainstorms set off multiple slope failures and flood damage across the nearly 70,000-acre park. By summer, almost every major park road had been closed. The park's new physical scientist, Jonathan Malzone, set out to learn what had happened, and why.

Why the risk is rising

• By 2100, the park is expected to warm 3.2–9.4 °F on average and get 8%–15% more precipitation a year.
• Storms are expected to grow more intense: the kind that used to come every 50 years may now come every 30.
• The invasive hemlock woolly adelgid has devastated native hemlock stands over the past 30 years. Leaf cover works like an umbrella, catching 10%–50% of annual rainfall; as evergreens give way to trees that drop their leaves, more rain — and harder rain — reaches the ground in winter.

A hidden pattern

The park had few written records of past failures. Malzone gathered them from news articles, government reports, geological publications, staff memories and his own observations — and found 25 roadside slope failures since 1881.

Why had no one noticed? The failures happened miles and years apart, and staff moved on. And almost all came with extreme rain, when floods, fallen trees and broken infrastructure kept crews busy with repairs, not geology reports.

A road slope failure that began in the spring 2022 rains, March 2022. NPS / Jonathan Malzone

The same site in December 2023; the failure kept moving through 2023 and 2024. NPS / Jonathan Malzone

Mapping 500 scars

Using high-resolution elevation data, Malzone built a digital elevation model of the park and found more than 500 slope-failure scars from across geologic time. With geologic maps, fieldwork and earlier studies, he listed six natural conditions that make slopes fail:

These are legacies of how the Appalachian Mountains formed and of the ice ages that buried the area under glaciers.

Next came fieldwork: monitoring stations and movement sensors at active failures, plus rainfall records from a park station run by the Western Regional Climate Center.

A crack sensor at a landslide that suddenly began moving after the 2022 rains. By March 2024 it had shifted more than a meter (three feet). NPS / Jonathan Malzone

With the NPS Geologic Resources Division, the park started an unstable slope management program — a rating system for quick field assessments. It flagged 37 slopes near roads and infrastructure that need regular monitoring; roads and trails that visitors depend on were the most often threatened.

The trigger: extreme storms

Comparing failures with rainfall records from 2004 to 2023, Malzone found that every known slope failure came during or just after a storm in the top 5% for intensity, duration and total rain.

Storms from 2004 to 2023 by total rainfall, duration and peak intensity; circled points are the four storms linked to known slope failures. NPS / Jonathan Malzone; data: Western Regional Climate Center

One rain gauge can't predict everything, but it gives a reasonable yardstick for weather alerts: now a forecast alone can warn staff when slopes are at risk.

Route 209: no quick fix

U.S. Route 209 runs along the Pennsylvania side of the park, beside the Middle Delaware National Scenic and Recreational River — a busy commuter road, emergency route and gateway to popular sites. At one spot it crosses a steep slope of loose shale 100–150 feet above the river, with the McDade Trail squeezed between, beside a deep eddy pool that is a historical landmark and a favorite fishing spot.

The Route 209 slope failure, March 10, 2022; afterward less than 8 feet of stable ground remained between the road shoulder and the slide. NPS / Kris Salapek

The 2022 storm left just 8 feet of stable ground between road and river, and the park closed the road — frustrating the public and cutting emergency and boater access. After consulting the Geologic Resources Division, the U.S. Geological Survey and the Federal Highway Administration, it reopened one lane with a temporary traffic light.

The research showed frequent failures here over the last century. Resource manager Kara Deutsch stressed investing in sustainable infrastructure that anticipates hydrologic and ecological change. But fixing the slope will affect trail and fishing access, cultural sites and scenery. The valley is at the center of the Lenape homelands of four federally recognized Tribes — the Delaware Nation, Delaware Tribe, Stockbridge-Munsee Community Band of Mohican Indians and Shawnee Tribe — and the park works with them through consultation and co-stewardship. With the NPS Denver Service Center, it is weighing options such as context-sensitive rock slopes, deep-rooted native plants and keeping winter leaf cover.

The lesson

Institutional memory isn't enough when the climate is changing. The unstable slope program now ties the data together in a permanent record — one that doesn't depend on anyone remembering the last record-breaking downpour.

Sources

Based on Jonathan Malzone, "Caught By Surprise, One Park Learned Its True Landslide Risk," Park Science, Summer 2024, National Park Service; a work of the United States government in the public domain.