These neighborhoods received fewer community investments, including interventions such as stormwater infrastructure and landscape modification, than did higher-wealth neighborhoods.

We looked at whether these decades-old decisions have had any impact on flood risk today and learned that they do.

For this study, we correlated present-day flood risk in metro Detroit with former Home Owner’s Loan Corporation boundaries’ grades. Flood risk was mapped using the First Street Foundation’s Flood Factor, which scores every parcel in the U.S. on a scale of minimal (1) to extreme (10).

We then correlated flood risk to the presence of ghost streams and wetlands, which we extracted from old topographic maps from the United States Geological Survey. The goal was to determine whether a history of waterway burial and/or redlining influenced the overall flood risk of communities today.

We found that flood risk was disproportionately distributed, with historically redlined neighborhoods bearing the greatest brunt of flood risk.

Residents living in communities that were graded as “hazardous” (D) or “declining” (C) in the 1940s are today more susceptible to flood risk than the more affluent A and B communities. Over 95% of parcels classified at extreme flooding risk occur in C and D communities, with less than 4% in A and B communities.

Flood risk increases with the presence of ghost streams and wetlands, with C and D communities having a higher risk. In C communities, the presence of a ghost wetland increases flood risk tenfold, while ghost rivers also increase risk, although by a smaller amount.

The percent of properties in D-graded communities that are located adjacent to the 32-mile-long Detroit River and classified at extreme or severe flood risk is 99.9% if they have ghost wetlands or 95% if they have ghost rivers.

In other words, the combined history of redlining and landscape alteration may still contribute to increased flood risk today. When communities received poor grades, banks, lenders and municipalities neglected those areas’ stormwater infrastructure.

If communities want to protect residents from flooding, it’s crucial for them to map and understand their “hidden hydrology.” Few cities have the data to inform residents that they are at greater flood risk because they are living on a ghost wetland or river.

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