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DNRC Headquarters
1539 Eleventh Ave. Helena, MT 59601
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Floodplain Mapping FAQs

General Questions

What are floodplains?

Floodplains are areas along a waterway that are prone to flooding.  Flooding can happen anywhere, but certain areas are especially prone to flooding from waterways.

What is a 100-Year Floodplain (1% Annual Chance Floodplain)?

A 100-Year Floodplain (1% Annual Chance Floodplain) is an area that will be inundated by a 100-Year Flood, a flood event having a 1% chance of being equaled or exceeded in any given year. In a 100-Year Floodplain, there is at least a 1-in-4 chance of flooding during a 30-year mortgage. The 100-Year Flood is also referred to as 1% Annual Chance Flood or a Base Flood.

The 100-Year Floodplain is the area designated by the Federal Emergency Management Agency (FEMA) and the State of Montana as a high-risk area for floodplain management and flood insurance purposes.

What is a floodway?

A floodway is the area within the 100-year floodplain that must be kept free from new development so that the 100-year flood (1% annual chance flood, base flood) can be carried without substantial increases in flood heights. The Floodway will usually see the deepest and fastest water during a 100-year event.

What is the 500-year floodplain (0.2% Annual Chance Floodplain)?

A 500-Year Floodplain (0.2% Annual Chance Floodplain) is considered to have a MODERATE flood risk.   It  is an area that is expected to be inundated by a 500-Year Flood, a flood event having a 0.2% chance of being equaled or exceeded in any given year. The 500-Year Flood is also referred to as a 0.2% Annual Chance Flood.


Floodplain Mapping Questions

Why are floodplains mapped?

Floodplains are mapped to accurately depict flood risk and understanding flood risk is the first step towards flood protection. Accurate floodplain maps help residents, businesses, and the County, Cities and Towns make informed decisions to ensure personal safety, protect financial assets, and plan for emergencies.

What is a Flood Insurance Rate Map?

The Federal Emergency Management Agency (FEMA) works with the State of Montana and its communities to conduct flood studies and depict flood hazards on FEMA flood maps, known as Flood Insurance Rate Maps.  Flood Insurance Rate Maps illustrate the extent of flood hazards in a community by depicting a variety of information.

Beaverhead County and the City of Dillon have had Flood Insurance Rate Maps since 1982. The Town of Lima has had a Flood Insurance Rate Map since 1986.  The County, City, and Town administer Floodplain Management Regulations in mapped 100-Year Floodplains to minimize flood losses and threats to public health, safety, and welfare.  The current Flood Insurance Rate Maps can be found on FEMA's Flood Map Service Center website:

How are Flood Insurance Rate Maps used?

Flood Insurance Rate Maps indicate areas of flood risk and they are used by different entities for a variety of reasons.

  • Beaverhead County, the City of Dillon, and the Town of Lima use Flood Insurance Rate Maps:
    • To regulate flood risk areas.
    • To inform local emergency planning and mitigation, land-use planning, and growth policies.
  • Property owners and developers use Flood Insurance Rate Maps for subdivision and planning purposes.
  • Lenders use Flood Insurance Rate Maps to determine if a structure is in an area of flood risk.
  • Insurance Agents use Flood Insurance Rate Maps to determine flood insurance policies and premiums.

Flood Insurance Rate Maps need periodic updating if an area has changed or if better information becomes available. 

Is there a difference between a "draft floodplain map" and a "Preliminary Flood Insurance Rate Map"?

Generally, there are minor differences between draft floodplain maps and Preliminary Flood Insurance Rate Maps. Draft floodplain maps are developed first. FEMA uses the draft floodplain map data to create Preliminary Flood Insurance Rate Maps. Preliminary Flood Insurance Rate Maps are very similar to draft floodplain maps, but with minor updates.

In addition, unlike draft floodplain maps, Preliminary Flood Insurance Rate Maps have been fully converted to FEMA’s Flood Insurance Rate Map format. Any official Appeals or Comments submitted to FEMA during the 90-Day Appeal and Comment Period will need to reference a specific Preliminary Flood Insurance Rate Map panel number. The tentative FEMA Comment and Appeal Period is estimated to be mid 2021.

Project Questions

Why are the existing Flood Insurance Rate Maps being updated?
Most of the existing Flood Insurance Rate Maps in Beaverhead County were delineated from studies completed in the late 1970s. New data and modern engineering methods are now being used to map the floodplains more accurately.  After a technical and public review process, the draft maps will eventually replace the existing Flood Insurance Rate Maps.

What measures are in place to ensure the information used for the update will be as accurate as possible?
High-accuracy topographic information, updated hydrologic data, and more modern engineering methods are being used to ensure the maps reflect the flood risk accurately.

Where can I see the proposed changes between existing Flood Insurance Rate Maps and the Preliminary maps?
The map viewer can be accessed here:  Preliminary Maps.  You can also view individual Preliminary Maps and Technical Reports here                                        

Insurance Questions

Will I be required to purchase flood insurance?

It depends. Lenders can require flood insurance for a building they are financing, regardless of where the building is located. For federally backed loans, lenders will use existing Flood Insurance Rate Maps and must require flood insurance when a building is in a mapped 100-Year Floodplain, unless it can be proven that the building is actually above the flood elevation or outside of the floodplain. Periodically, lenders will review their loans to reassess flood risk based on the best available data. Flood Insurance Rate Map updates will typically trigger a periodic review.

Some property owners may not see any insurance changes with this mapping update. For areas that will be newly mapped into the floodplain, a lender may require flood insurance and/or elevation documentation for a building. If you own a building in an area that will be newly mapped into a 100-Year Floodplain, contact your lender as early as possible to start the discussion about flood insurance.

Can I get a subsidy on my flood insurance premiums if my property is newly mapped into a 100-Year Floodplain?

Yes. Buildings in newly mapped areas are eligible for subsidized premiums as long as flood insurance is purchased within 11 months of the date the new Flood Insurance Rate Maps become effective (estimated for 2022). If you own a building in an area that will be newly mapped into a 100-Year Floodplain, contact your lender as early as possible to start the discussion about flood insurance.

Am I required to purchase flood insurance even though my building is not (and will not be) shown in a 100-Year Floodplain?

It depends. A lender does not need to require flood insurance when a building carrying a federally backed loan is located outside of a 100-Year Floodplain due to a decreased flood risk. A lender may still require flood insurance regardless of a building’s location, however, in order to protect their investment.

Every building has a risk of flooding even if it is not located in a high-risk, 100-Year Floodplain. It is therefore recommended that you purchase flood insurance even if your lender does not require you to do so.

Development Questions

I want to develop my property. How could the proposed floodplain changes affect me?

Beaverhead County, City of Dillon, Town of Lima have floodplain regulations that apply to areas located in the 100-Year Floodplain. The County, City and Town require a floodplain permit for any development project in a mapped floodplain.

Preliminary maps are going through the review process and are not yet effective. Until these maps become effective, the communities strongly encourage proposed construction projects on properties newly mapped into a higher-risk flood zone to utilize preliminary map data to ensure that property development methods will minimize flood damage. Preliminary map data also provides an opportunity to develop smarter and thereby reduce potentially high costs of flood insurance premiums.

It is important to discuss the Floodplain Regulations with county, city, or town staff to understand the impacts of Flood Insurance Rate Map updates on proposed construction projects in a mapped floodplain.



More Info

Maps & Technical Reports
Floodplain FAQs

Contact the County

Rob Macioroski
County Floodplain Administrator
(406) 683-3768

Contact the City of Dillon

Todd Hazelbaker
City Floodplain Administrator
(406) 683-4245

Contact the Town of Lima

Dina Young
Town Floodplain Administrator
(406) 276-3521

Contact DNRC

Tiffany Lyden
Outreach Specialist
(406) 444-0599