Flood Zones in Delaware County, Iowa

A flood record set in 2004, then broken again in 2010

The Maquoketa River at Manchester has two separately documented, quantified flood peaks worth knowing before buying land near it. On May 23, 2004, intense thunderstorms across northeast Iowa produced a peak discharge of 26,000 cubic feet per second at the Maquoketa River at Manchester streamgage -- a roughly 100-year event, and at the time, the largest known flood in the upper part of the Maquoketa River basin, according to USGS's own reporting. Older, less precisely documented floods go back further still: a 1925 flood caused extensive damage in Manchester, and additional historic floods are recorded for 1944 and 1947, though detailed discharge figures for those earlier events are limited.

That 2004 record stood for only six years. Major regional flooding hit the Maquoketa River basin again on July 23-26, 2010, and on July 24 specifically, the privately owned Lake Delhi dam -- located on the Maquoketa River upstream of Manchester -- breached. USGS's reporting on the event describes the dam breach as having aggravated flooding that was already underway from the storm itself, rather than being flooding's sole cause. The combined effect pushed the Maquoketa River at Manchester to a peak discharge of 26,600 cfs on July 24, 2010 -- narrowly surpassing the 2004 event, and standing as the largest flood on record for that gauge as of USGS's reporting.

What's confirmed, and what genuinely isn't, about current conditions

It's important to be direct about the limits of what this research could confirm. The 2004 and 2010 flood events themselves, their discharge figures, and the basic fact that a dam breach compounded the 2010 flooding are all independently confirmed through USGS's own published reporting on each event. What this research could not confirm, because direct access to government and reference sources was blocked during this research pass and the available search budget was exhausted before a targeted follow-up could be completed, is the current, 2026 status of the Lake Delhi dam and the lake it once impounded -- whether the dam has since been repaired or rebuilt, what entity (if any) currently manages the impoundment, what the lake's current water level or shoreline extent looks like, and what dam-safety or floodplain restrictions currently apply to land around it.

This gap matters specifically for anyone evaluating a parcel on or near the former Lake Delhi impoundment, since a rebuilt, full lake and an unrepaired, drained impoundment are two very different physical and regulatory situations for shoreline or near-shoreline property. Do not assume either outcome -- confirm the dam and lake's actual current status directly before evaluating any specific parcel near it.

• Pull the current FEMA Flood Insurance Rate Map for any Delaware County parcel near the Maquoketa River, whether upstream or downstream of the former Lake Delhi dam site, rather than relying on this page's historical flood figures alone.
• For any parcel specifically on or near the Lake Delhi impoundment, contact Delaware County Zoning and Iowa DNR directly to confirm the dam's current physical and regulatory status, the lake's current extent, and any applicable dam-safety setbacks or floodplain restrictions before making an offer.
• Ask a seller directly whether a specific property experienced flood damage during the July 2010 event, and request any available documentation -- a property with a 2010 claims or damage history carries different insurance and disclosure implications than one without.
• If the property is near the dam site itself, ask whether any dam-safety inspection reports or regulatory correspondence exist and are available for review, given the dam's documented failure history.