Impact of Tropical Storm Isaias on the Wissahickon Watershed

Jamie Stewarti
By Madalyn Neff, Communications Specialist


On August 4, Tropical Storm Isaias moved through the Wissahickon Valley—affecting our homes, roads, and businesses. We felt its impacts deeply at Wissahickon Trails. We experienced damaging floods at the Four Mills Barn (our offices), Evans-Mumbower Mill, and along the length of the Green Ribbon Trail. We are still learning the full extent of the damage and its cost, but as many of our friends and supporters have expressed concern about the flooding and offered their support, we wanted to provide you with an update.

The Extent of the Damage

Four Mills Barn

The Four Mills Barn, our headquarters, took on about 10 inches of water, heavily damaging the first floor. Files, photos, and furniture were engulfed in muddy water and could not be salvaged. Our copier and HVAC systems were submerged and suffered damages as well. We are thankful to the friends and family that offered fans and dehumidifiers on such short notice—these appliances almost certainly prevented more damage. We are working with flood specialists for a thorough cleaning and mold abatement treatment. 

iJamie Stewart

Green Ribbon Trail

This beloved trail follows the Wissahickon Creek, and when the creek jumped its banks, the trail became submerged. The powerful floodwaters left not only debris, but destruction of our built structures, namely boardwalks, footbridges, stepping stones, and signage. As a result, we immediately closed the trail for the safety of our community. Our conservation team and several volunteers conducted assessments and we reopened sections of the trail that have been identified as safe. We do not yet know the cost of repairing and replacing the boardwalks and stepping stones, but we know that they are essential for both trail users and the habitat.  

iMargaret Rohde

Evans-Mumbower Mill

The Mill had about 24 inches of standing water in the basement at its peak. The good news is that this water drained quickly and completely and there is little to no water damage. One window pane was broken and there was a layer of silt that we have cleaned up from the floorboards. 

iMark Smith
We Need Your Help



We are still learning the full financial impact of the flooding on the Green Ribbon Trail and Four Mills Barn. But we can conservatively say it will be in excess of $30,000.  


At the barn, we have contracted with a cleaning and remediation service that has removed about 18 inches of damaged drywall and insulation from the entire ground floor, treated mold and mildew, and generally made the space safe, healthy, and inhabitable again. 


For the Green Ribbon Trail, we hope to salvage and repair what we can. But we may need to build new boardwalks and footbridges to ensure they are structurally sound. A total of 5 stepping stones (worth about $10,000) have been uprooted.  


If you are able, please donate today to help us recover from the flood.  



We are so grateful for all of the support and offers of volunteer service in the aftermath of the storm as we cannot clean and repair the Green Ribbon Trail alone. We are planning a socially distanced Creek Cleanup in September, please join us in cleaning the trails and creek. In the meantime, we are working in small groups with our Conservation Crew on trail repairs and to rebuild boardwalks and other structures.

Green Ribbon Trail Updates


[9/15/20] All sections have reopened. Note: The Route 202 crossing is still closed due to previously scheduled construction, which is labeled on this Google Map


[8/28/20] With the exception of two stepping stone crossings (Rubenstein & Lafayette), the Green Ribbon Trail is fully reopened. Please be aware that some bridges or boardwalks may still be out of place, but all sections are now open and safe for trail users to enjoy. 


[8/20/20] The Green Ribbon Trail sustained damage from Tropical Storm Isaias, necessitating the closing of many sections due to safety concerns and impassable obstacles.  The sections which remain open include:

  • Trailhead in Upper Gwynedd Township at Parkside Place to North Wales Road

  • Walk-in access point near Upper Gwynedd Wastewater Treatment Plant (off of Township Line Road) to the Runner's Crossing

  • Plymouth Road entrance to Penllyn Blue Bell Pike

  • Old Penllyn Pike to Mt. Pleasant Ave

  • Butler Pike to the bridges at Four Mills Nature Reserve


We want to keep as much of the trail open as possible, but please be aware that these areas took a hit, too, and some bridges and boardwalks may be out of place, there may be a small number of downed trees and debris to step over, as well as some slippery, muddier-than-usual spots. We believe these sections are safe, but trail users should use extra caution until we can fully restore them. Staff  and volunteers will be working hard over the next few weeks and beyond to get the rest of the trail repaired and open.


[8/4/20] Flash flooding conditions in the Wissahickon Creek have caused considerable damage along the Green Ribbon Trail and we have closed it for the safety of our community. Stay tuned for more updates.

Jamie Stewarti
Stormwater Management is Vitally Important 

On Tuesday, 8/4/20 we saw severe flooding across the Wissahickon watershed. While Tropical Storm Isaias affected our homes, roads, and businesses, this was just a symptom of a problem we face every day – stormwater. Development and suburban sprawl are not new to our region, but each time more and more impervious surfaces like parking lots and buildings are added to our landscape, water has fewer and fewer places to go when it rains. 


Wissahickon Trails has been working with community partners for over 63 years to protect the creek and its tributaries, preserve land to slow the flow of stormwater, and work together with municipalities and homeowners to find new solutions. This week, we saw how critical this work is for the health and safety of our community as we face the realities of climate change in our region. We need your help to keep going. Whether you volunteer, implement stormwater solutions on your property, donate, or ask your municipality to fund stormwater management--every action makes a difference.

Learn more about our Water Initiatives.