There are plenty of things to think about when developing land near a river. Consideration of these issues before you buy or build will help prevent unexpected expenses and challenges. Choosing the right location before you build is far easier and cheaper than dealing with flood and erosion emergencies.
Rivers in Montana are wild and do not always stay within their banks. During the spring melt or after intense rains Montana’s rivers periodically spill over their banks into the adjacent land, or the floodplain. To avoid getting wet, locate buildings and permanent structures well outside the 100-year floodplain, which is the land that has a 1% change of flooding in any given year. Learn about flood insurance.
- Before you build, find your 100-year floodplain and check local regulations
- Build outside the floodplain to protect your investments from flooding
- A common misconception is that a 100-year flood happens every 100 years. 100-year floods have a 1-in-100 chance of happening in any given year.
How can I find out where the floodplain is?
- FEMA provides official floodplain maps
- Your city or county planning department may have floodplains maps and recommendations
- If you need help finding floodplain maps in your area, talk to your local conservation district
Example of 100-year floodplain along the Yellowstone River.
To avoid risk of flooding, build outside the floodplain.
Choosing to build outside the floodplain will reduce the need to build expensive and environmentally damaging flood control structures. While it may be inconvenient for us, flooding is an important mechanism for maintaining river health. Floodplains spread and store water during high flows, reducing damaging erosion and further flooding downstream. Floodwaters exchange nutrients, sediments, and organisms between the channel and the floodplain, creating unique habitats that are critical for the survival and reproduction of many species.
As a homeowner, you are responsible for maintaining your septic system. Proper design, construction, and maintenance of a septic system are essential for the safety of your health and the environment.
The EPA lists four things you can do protect your septic system:
- Inspect your system every year and pump the tank every 3-5 years or as necessary
- Use water efficiently
- Don’t dispose of household wastes in sinks or toilets
- Care for your drainfield
Septic systems in Montana must meet state, city, and county regulations. Some regulations include having a drain field at least 100 feet away from the nearest 100 year floodplain and the bottom of the drainfield 4 feet above the 100 year flood elevation. Get in touch with your county health/sanitation department to learn about requirements and permits that you will need to install a new septic system.
Guides and Resources:
Additional things to watch out for to avoid future problems:
- Steep or vertical bank without vegetation cover: Vegetation protects banks from erosion. A steep bank without vegetation is undergoing active erosion and indicates channel movement.
This bank is bare of vegetation and being eroded. The grassy area behind the bank is not safe for development, as it will soon be threatened with erosion.
Maintaining robust vegetation cover – shrubs and trees, not lawns – along the river bank should always be your first choice in protecting your property from erosion. Lawns are not sufficient to prevent erosion.
- Evidence of downslope movement of soil: both along streams and in upland areas
Downslope movement of soil (sloughing) indicates that an area is undergoing active erosion.
- Ice jams: Rivers freeze in Montana! Ice jams are sudden floods caused by ice buildup on rivers causing the water to backup into the floodplain. Talk to your neighbors and your conservation district to learn if your area is prone to ice jams. The dangers of ice jams further reinforce the importance of building at a safe distance away from the river bank. This website maps ice jams across the state and has more information.
- Outside bends: River bends are where erosion and deposition are actively occurring. Outside bends tend to propagate down the channel. Building near an outside bend is hazardous and should be avoided.
Montana may not be as famous for earthquakes as its neighbors to the west, but it is one of the top 10 most seismically active states. The largest earthquake in recent history occurred in 1959. The magnitude 7.5 Hebgen Lake earthquake set off a landslide of 58 million cubic yards of earth that dammed the Madison River, killed 28 people and created Quake Lake. Such large earthquakes are rare, but 7-10 small earthquakes occur every day in Montana.
Information on earthquake preparedness can be found through the following links.
Montana is prone to prairie and forest wildfires, that can destroy homes and property. A key concept for homeowners is defensible space, modifications to landscape and vegetation surround a structure to reduce the likelihood of ignition. In the event of a large wildfire, firefighters will not be able to actively defend every home. A buffer zone around your house will increase the likelihood of it surviving the fire.
Fire Safe Montana has developed guides for fire resistant construction
Understand that building and living in forested areas carries a risk of wildfire. Use this Montana DNRC guide to evaluate the fire risk to your current or future home.
Wildfires can increase the risk of flooding by destroying vegetation that absorb rainfall and hold the soil in place. Burned zones are at a higher risk for flooding.
Rivers move! A healthy river maintains a balance between eroding and depositing sediments. Over time, this process leads to the river channel migrating across the river valley bottom, threatening any structures and developments in its path.
- Understand that rivers are dynamic and move over time – an area may look safe for development now but in ten years it may not be
- Locate structures outside the channel migration zone
- Outside bends of rivers are most vulnerable to erosion
Thankfully, experts can predict where rivers are likely to move in the next hundred years and have created maps called channel migration zones (CMZs). For the long-term safety of your investment, locate buildings and permanent structures outside the CMZ. Many rivers in Montana, like the Yellowstone and Musselshell, have completed CMZ maps. Contact your local conservation district to obtain CMZs for your area.
Keeping buildings outside the CMZ will reduce the need for costly bank armor. Bank armor harms fisheries and ecological health by interfering with erosion and deposition processes. Bank armor has local benefits, however it prevents the dissipation of energy, intensifying downstream floods and erosion for your neighbors.
Outside of a municipality or subdivision you will be responsible for providing water. The most common sources of water are groundwater wells.
Do not assume that groundwater will always be easily available. Before buying a property that lacks a well, you will need to determine the groundwater availability at the site. Talk to the neighbors, local well drillers, and the county planning board. They have intimate knowledge of the local groundwater conditions. The Montana Bureau of Mines and Geology has well logs for local groundwater developments and is a wealth of expertise and advice on all subjects relating to wells and groundwater. Once the well is developed, you will need to file a Notice of Completion with the Montana Department of Natural Resources and Conservation to obtain a water right for the well.
Water well drilling for the prospective owner provides detailed information on drilling a new well.
Montana follows the Prior Appropriation/Western Doctrine for water rights, where the right to use water is determined by historic use and availability. Water rights are established by putting water to beneficial use, and the earliest water rights have priority in times of shortage. Water rights may or may not be attached to the property when you buy it, so check with the local Montana DNRC Regional Office to see if you can use the water and how much. There are many examples of homeowners along Montana rivers illegally putting small pumps in the river to water their lawns without water rights. When in doubt, do not use river water until you are sure that you can legally do so.