Three gauges, and they disagree about the worst day
| Gauge | Flood stage | Moderate | Major | Record crest |
|---|---|---|---|---|
| Schuylkill at Norristown | 13 ft | 17 ft | 19 ft | 26.85 ft, 2 Sep 2021 |
| Schuylkill at Pottstown | 12.5 ft | 18 ft | 22 ft | 29.97 ft, 23 Jun 1972 |
| Perkiomen at Graterford | 11 ft | 13 ft | 16 ft | 24.23 ft, 2 Sep 2021 |
All three are National Weather Service gauges over USGS stations, read on 26 August 2026. Same county, same river in two of the three cases, and two different storms hold the records.
Norristown: the record is 2021, and it keeps company
The Schuylkill at Norristown crested at 26.85 feet on 2 September 2021 during Ida, at 143,056 cubic feet per second. That broke 25.10 feet from 23 June 1972, Agnes, which had stood for forty-nine years.
The rest of the list is the part worth reading twice: 22.0 ft in September 1999 (Floyd), 21.0 ft in August 1933, 20.81 ft in May 2014, 20.55 ft in August 2020 (Isaias), 19.76 ft in August 2011, 19.3 ft in September 1971, 19.13 ft in June 2006, and 19.0 ft in January 1996.
Major flood stage at Norristown is 19 feet. Count the entries above it since 2006 and you get five — 2006, 2011, 2014, 2020 and 2021. Five in twenty years is a pattern, and it is why the flood question in Norristown comes before the price question. Stony Creek and Saw Mill Run add their own water to it. Anything on the river side of Main Street, and anything along Stony Creek, wants the FEMA map pulled and a flood quote in hand before an offer goes in.
Pottstown: the record is still Agnes, and Ida barely registered
Upstream at Pottstown, the record crest is 29.97 feet on 23 June 1972, Agnes, at 95,900 cubic feet per second — and it is not close. Second place is 21.0 feet from February 1902. Then 20.84 ft in June 2006, 20.15 ft in May 1942, 19.2 ft in August 1933, 18.27 ft in September 2011, 18.21 ft in January 1979 and 17.98 ft in August 1955 during Diane.
Ida did not come near it. The Delaware River Basin Commission reported only minor flooding at Pottstown while Norristown and points downstream took the major flooding. So the two Schuylkill towns run opposite to each other: upstream Pottstown’s worst day is 1972, downstream Norristown’s worst day is 2021.
Pottstown’s own water problem is local anyway. The USGS gauge sits three tenths of a mile downstream of the mouth of Manatawny Creek, and Manatawny, Goose Run and Tanyard Run are the borough’s own drainage story. Stormwater arch repairs on Goose Run and Tanyard Run were funded at $548,450 from state water programs, with construction expected to begin in August 2026 and to run four to five weeks.
Collegeville, and what the Perkiomen did
The Perkiomen gauge at Graterford, just upstream in Perkiomen Township, set its record at 24.23 feet on 2 September 2021 at 45,274 cubic feet per second. Second is 19.32 feet from August 2020, Isaias, at 44,613 cubic feet per second. Then 18.26 ft in July 1935, 18.2 ft in August 2011 during Irene, and 17.08 ft in June 1972 — Agnes is only fifth on this creek. The two worst days on record here happened thirteen months apart.
Read the 24.23 with a caveat the borough itself supplies: the gauge tops out near 24 feet, so it was at its limit. The borough describes the creek going off the charts with a peak flow near 70,000 cubic feet per second, and a secondary account estimates a crest of 26.5 feet. The published number is 24.23, and the honest sentence is that the instrument ran out of scale.
What that did inside the borough comes from Collegeville’s own report, delivered as testimony by Borough Council President Cathy Kernen and Crystal Gilchrist to the Pennsylvania House Democratic Policy Committee on 3 October 2022: more than 20 borough homes destroyed, Main Street under water from First to Second Avenues, six businesses flooded, traffic signals knocked out, and the early-1700s Perkiomen Bridge Hotel extensively damaged. Across the watershed, 75 homes destroyed and two deaths. The report’s ask was state-funded, watershed-scale stormwater studies under an adapted Act 167.
One thing that report does not describe is a buyout program, a stormwater fee or a FEMA map revision, and nobody should imply any of the three exists.
What flood risk actually does to a mortgage
The FEMA map decides it, and it decides it about the structure, not the town. Where a building sits inside a Special Flood Hazard Area and the loan is federally backed or federally regulated, flood insurance is required for the life of the loan and it is escrowed alongside real estate taxes and homeowner’s insurance.
That is the mechanical consequence, and it is the one people miss: the premium lands inside the monthly figure the lender is working with, next to a tax line already built from real millage. We quote no premium here, ever. An insurance agent and the lender who calls you back price it against the actual flood zone, the elevation and the coverage.
An elevation certificate is the document that can move it. A surveyor records the elevation of the lowest floor against the base flood elevation, and on a borderline property that certificate is what changes the rating. It is worth ordering while inspection time is still running, because the same document costs the same money and buys far less leverage after an appraisal has come back.
And maps move. A floodplain map revision was approved on 18 March 2026 for a Lansdale site at Fifth Street and Valley Forge Road, where a stone-walled channel crosses the ground with no existing stormwater controls. Horsham Township acquired a 1.77-acre parcel on McKean Road through FEMA’s Repetitive Damage Program after Ida and a 2021 tornado, and holds it with no public access. The current map governs, and the current map is the one to pull.
Away from the two big rivers the question is quieter and the answer is the same. Whitpain Township sits on a divide with no gauge history at all. Plymouth Meeting sits on the ridge above the Schuylkill, with the floodplain exposure on the Spring Mill river-flat edge of Whitemarsh and along Plymouth Creek. And on 5 August 2026, I-76 eastbound flooded near the Green Lane Bridge in the Bala Cynwyd stretch around 9:15 in the morning and reopened by about one o’clock. Creek lots need the map like river lots do.
What to do next
Get the flood determination for the structure before you fall for the porch, and if it comes back inside a hazard area, get an insurance quote and an elevation certificate while you still have inspection time. Then call with the address and what the map says, and one licensed Pennsylvania mortgage lender calls you back — usually the same working day, and by the end of the next working day at the outside. Or put the address in the form and send it along. A flood line found in week one is a negotiation; found in week five it is a scramble.