Flooding

Flooding is an overflow and inundation of water onto normally dry land as a result of:

  1. the overflow of inland or tidal waters, or
  2. the unusual and rapid accumulation or runoff of surface waters from any source.  

On this page you will find information on the various types of flooding that may occur in Maine and notable historic floods in the state.

Inland Flooding

Inland flooding occurs when moderate precipitation accumulates over several days, intense precipitation falls over a short period, there is abundant runoff from spring snowmelt, a river overflows because of an ice or debris jam or dam or levee failure, or a combination of these factors. The following flood mechanisms occur during inland flooding: 

Riverine Flood

A river flood occurs when water levels rise over the top of the riverbanks due to excessive rainfall from low pressure systems, landfilling tropical systems, persistent nearly stationary thunderstorms over extended periods of time, or a combination of snowmelt and rainfall along with ice jams. Periodic overbank flow of rivers and streams is a typical result of spring runoff in Maine. See “Location of River Basin” section for flooding details.  

Lacustrine Flood

Lacustrine or lake flooding occurs when the outlet for the lake cannot discharge the flood waters fast enough to maintain the normal pool elevation of the lake.  During a base flood event, normal increases in water surface elevations on most Maine lakes and ponds range from 1 to 5 feet. However, in Maine there are some examples where the base flood event will reverse the flow of the outlet stream. In such instances, river and base flood elevations can rise more than 15 feet above normal pool. Maine’s mandatory shore land zoning and floodplain management elevation requirements do much to mitigate lake and pond development by imposing significant setbacks from the water’s edge. This type of flooding can impact private camps built near the water’s edge. Though less common than riverine floods, there is documented damage from lacustrine flooding in Aroostook County in 2018.

Ice Jams

Ice jams occur when warm temperatures and heavy rain cause snow to melt rapidly. Snow melt combined with heavy rains can cause frozen rivers to swell, which breaks the ice layer on top of the river. The ice layer often breaks into large chunks, which float downstream and often pile up in sharp river bends, shallow river channels, mouths of tributaries, points where river slope decreases, and near narrow passages around other obstructions such as bridges and dams.  The channel blockage acts like a temporary dam causing the water to rise rapidly behind the jam causing a rapid onset of upstream flooding. If the ice jam suddenly breaks, a torrent of water is rapidly released downriver causing flash flooding below the jam location. Damages from ice jam flooding usually exceed those of clear water flooding because of higher than predicted flood elevations, rapid increase in water levels upstream and downstream, and physical damage caused by ice chunks. Moving ice masses can shear off trees and destroy buildings and bridges above the level of the flood waters.

Flash Flood

A flash flood is caused by heavy or excessive rainfall in a short period of time, generally less than 6 hours. Flash floods are usually characterized by raging torrents after heavy rains that rip through riverbeds, urban streets, or mountain canyons sweeping everything before them. They can occur within minutes or a few hours of excessive rainfall. They can also occur even if no rain has fallen, for instance after a levee or dam has failed, or after a sudden release of water by a debris or ice jam. Flash floods are very dangerous and destructive not only because of the force of the water, but also the hurtling debris that is often swept up in the flow.

Coastal Flood

A coastal flood, or the temporary inundation of low-lying land areas along the coast, is caused by higher-than-average astronomical tide and is worsened by heavy rainfall, storm surge driven by onshore winds (i.e., wind blowing landward from the ocean), damaging waves, and sea level rise. Coastal flooding comes with two significant components: an increase in still-water levels and storm surge. The typical high winds associated with coastal storms exacerbate flooding by “pushing” more water toward land and increasing base water levels, or still-water levels. Strong storms such as tropical cyclones or nor’easters can cause large damaging waves and storm surges along areas of the coast of Maine. Fetch, or the distance the wind can blow over open water, is a significant factor in the size of storm waves. The shape of the ocean floor just offshore is another variable. The following flood mechanisms contribute to coastal flooding:

High Tide

High astronomical tides are produced in the ocean waters by the "heaping" action resulting from the horizontal flow of water toward two regions of the earth representing positions of maximum attraction of combined lunar and solar gravitational forces. Low tides are created by a compensating maximum withdrawal of water from regions around the earth midway between these two humps. The alternation of high and low tides is caused by the daily (or diurnal) rotation of the earth with respect to these two tidal humps and two tidal depressions. High astronomical tides are the highest levels that can be predicted to occur under average meteorological conditions.

Storm Surge

Storm surge is an abnormal rise in water level in coastal areas, over and above the regular astronomical tide, caused by forces generated from a severe storm's wind and low atmospheric pressure. Storm surge is extremely dangerous because it is capable of flooding large coastal areas. Extreme flooding can occur in coastal areas particularly when storm surge coincides with normal high tide, resulting in storm tides (see below). Along the coast, storm surge is often the greatest threat to life and property.

Storm Tide

Storm tide is a combination of predicted astronomical tide and storm surge.  It is the overall water level achieved during a storm event and is usually measured at a tide gauge.  For example, if a predicted astronomical tide is 10 feet, and 4 feet of storm surge comes in on top of that high tide, the storm tide level would be 14 feet.

Waves

Wind-driven waves, or surface waves, are created by the friction between wind and surface water. Generally, the larger the fetch (or the distance across open water that wind can blow), the larger the wave height. As wind blows across the surface of the ocean or a lake, the continual disturbance creates waves. As the wind blows for extended periods of time and over large distances, the wave heights increase.

Sea Level Rise

Global sea level rise is an increase in the world’s ocean’s surface height due to two dominant factors: volumetric increase and thermal expansion. Melting glaciers and land-based ice sheets, such as the Greenland ice sheet, which are linked to changes in atmospheric temperature, can contribute significant amounts of freshwater input to the Earth's oceans, increasing the volume of the oceans. Additionally, a steady increase in global atmospheric temperature creates an expansion of sea water molecules, thereby increasing ocean volume through thermal expansion. The Intergovernmental Panel on Climate Change Report estimates that the global sea level rise was approximately 1.7-1.8 millimeters per year (mm/yr) over the past century, based on tide station measurements around the world. Since 1993, satellites have measured average global sea levels and shown that the rate has increased to about 3.3 mm/yr (reference: U. Colorado). Climate models show that sea levels will continue to rise, with the 2017 US National Climate Assessment concluding that it is very likely to rise between 1 and 4 feet by the end of the century. Relative sea level rise, or local sea level rise, refers to how the height of the ocean changes relative to the land at a particular location. In Maine, there are four long-term tide gauges monitoring local sea levels. 

Long-term sea level trends in Maine indicate about half of the observed sea level rise has occurred since 1990, and rates are generally at or slightly above global long-term and short-term averages.  The Maine Climate Council recommends managing for 1.5 feet of relative sea level rise by 2050 and 4 feet by 2100. The Maine Geological Survey maintains a monthly Sea Level Rise Ticker and Dashboard for keeping tack of local sea level trends over the past century, based on tide station measurements around the world, with projected increased trends in sea level in the 20th Century based on global climate models. 

Other Flood Types

The following are additional flood types that may not necessarily be caused by typical rainfall, river flow, or coastal conditions. 

Urban/surface water flood

Surface water floods occur when an urban drainage system is overwhelmed, and water flows out into streets and nearby structures. Flooding from surface runoff can happen within minutes or more gradually, while the level of water is often shallow (rarely more than 1 meter deep). It creates no immediate threat to lives but may cause significant economic damage. The combined sanitary and storm water systems that some urban areas installed years ago cause flooding of sanitary sewerage when riparian or coastal floods occur. Runoff is increased due to many impervious surfaces such as roof tops, sidewalks, and paved streets.

Tsunami

A tsunami is a series of extremely long waves caused by a large and sudden displacement of the ocean, usually the result of an earthquake below or near the ocean floor. This force creates waves that radiate outward in all directions away from their source, sometimes crossing entire ocean basins. Unlike wind-driven waves, which only travel through the topmost layer of the ocean, tsunamis move through the entire water column, from the ocean floor to the ocean surface. Over 80% of tsunamis are caused by earthquakes on converging tectonic plate boundaries. Other causes include landslides, volcanic activity, certain types of weather, and—possibly—near-earth objects (e.g., asteroids, comets) colliding with or exploding above the ocean. Once a tsunami forms, its speed depends on the depth of the ocean. In the deep ocean, a tsunami can move as fast as a jet plane, over 500 mph, and its wavelength, the distance from crest to crest, may be hundreds of miles. All areas with elevation less than 100 feet and within two miles of the coast may have the very unlikely chance of tsunami impacts. Based on information obtained from the Maine Geological Survey, the chances of a catastrophic event impacting the Maine coastline are minimal. Tsunami modeling from the University of Rhode Island indicates the possibility of 5 to 6 meter waves along the coast of Maine if submarine landslides occur along the U.S. Continental Shelf. Maine is relatively protected from distant tsunami sources in the Azores and Caribbean, but local submarine landslides could produce waves reaching the coast of Maine. 

Dam Failure/Breach

Any malfunction or abnormality outside the design assumptions and parameters that adversely affect a dam’s primary function of impounding water is considered a dam failure. Lesser degrees of failure can progressively lead to or heighten the risk of a catastrophic failure, which may result in an uncontrolled release of the reservoir and can have a severe effect on persons and properties downstream. Dam breaches can cause rapid and expansive downstream flooding, loss of life, damage to property, and the forced evacuation of people. A dam breach has a low probability of occurring, but with a potentially high.

For more information, visit Maine's Dam Safety Program. 

Notable Floods in Maine

Flood of Record: The Great Flood of 1936

The flooding on March 19, 1936, was significant throughout southwestern and central Maine. The Kennebec, Androscoggin, and Saco River basins experienced the worst of the flood damage in Maine. According to the gaging station on the Androscoggin River at Auburn, the peak discharge was 135,000cfs, the largest discharge recorded at that site. Similarly, the peak discharge of the Mattawamkeag River near Mattawamkeag was the highest on record.

The meteorologic and soil conditions from the early winter season to just before March 19 were instrumental factors in the large discharges of the flood. In the early winter, the ground had frozen and was almost impermeable. Through January and February, many river basins of the State accumulated significant quantities of snow that created deep snowpack. The first warning sign came when warmer weather around March 9 began an early spring thaw. During the following 10 days, the Northeast experienced 2 major storms that only exacerbated the snowmelt and ice melt. 

The first of the major storms, the March 11-12 storm coincided with the breakup of thick ice that had formed on streams during the winter months. Streamflow records indicate the runoff from this first storm was about equal to the rainfall: thus, snowmelt didn’t contribute much to discharge after the first storm. While snowmelt was insignificant in the March 11-12 storm event, streamflow records report they had a much larger role in discharge during the second storm. Snowmelt, as well as the severe rainfall of the second storm, combined to release sizeable flows into already swollen river systems. Peak discharges after the second storm were far greater than those of the first storm.

While snowmelt and rainfall combined to dramatically elevate the water levels, large ice jams also played a major role in the heightening flood levels and damage. “Elevated river stages in Augusta and Hallowell, caused by ice jams, were 3.6 feet higher than the previous high-water records from March 2, 1896” (Maloney and Bartlett, 1991, p. 313). Another notable ice jam formed in a reach several miles long on the Androscoggin River just upstream from the pond of the powerplant above Lewiston. “According to powerplant records, this ice jam broke on March 20 and released a large volume of water that caused a rise of 1.75 feet in the pond in less than one-half hour” (p. 313). Those are just a couple of examples of the massive influence the ice jams had on increased flood levels.

When the ice jams released, the resulting ice flows compounded damage on several rivers by crashing through buildings and bridges downstream. Overall, the flood and ice floes destroyed or damaged 81 highway bridges. That is just one metric that highlights the immense damage the flood caused. In the aftermath of the flooding, five lives were lost, and property damage reached about $25 million. The one saving grace in this flood event was the timely warnings delivered to the public. Because the telephone, telegraph, and radio services kept the public advised about the severity of the floods well in advance of the flood crests, the loss of life was considerably lower than it could have been.
 

Flood of Record: The "April Fools Flood of 1987"

Records of past floods indicate that the April 1987 flood was one of the most significant in Maine’s history.  At selected sites, it was the worst since the area was settled more than 200 years ago. Flood damage in the Penobscot and Kennebec River basins in 1987 was the greatest for any flood (including March 1936) for which data is available.

Hydrometeorology conditions before the April 1987 flood gave no clear indication of the severity of the flooding that was to come. From December 1986 through March 1987, precipitation was below normal. In early March, the snowpack was below normal in northern Maine, normal in southern interior sections and above normal in coastal areas.” However, as spring approached, climatic conditions began to change and set the stage for trouble. March temperatures had finally gone above freezing, and then above normal, rapidly melting off the snowpack.  Runoff was then above normal in upland areas of western Maine.  From March 20 through April 2, multiple areas of low pressure moved slowly northeast toward Maine, bringing two storms that unleashed heavy rains. The resulting floods had only one missing factor – ice.  Had there been ice jams, the damage would have been far worse. “In contrast to the 1936 flood, during which backwater from ice jams was common, peak stages for the 1987 flood reflect primarily free-flowing conditions. 

Still, the damages were far reaching, affecting 14 of the 16 counties and a wide range of enterprises.  At least 2,100 homes were flooded, 215 were destroyed and 240 suffered major damage. Roads and bridges were destroyed. Many businesses had waterways instead of streets. Even in the first estimations, the Small Business Administration thought that 400 businesses had sustained losses totaling approximately $36,000,000. The Agricultural Stabilization and Conservation Service reported $300,000 worth of equipment and $100,000 in livestock losses.  Pollutants in flood waters contaminated clam beds at the mouth of rivers, putting clam diggers out of business. That alone necessitated Disaster Unemployment Assistance funding of over $300,000. 

According to MEMA accounting records, the “April Fool’s Flood” of 1987 was a $100,000,000 event. Were it to happen today, nearly 20 years later, the costs would be much higher, primarily because real estate and infrastructure values have continued to rise.
 

Flood of Record: The 2007 "Patriot's Day Storm"

According to the Gulf of Maine Ocean Observing System website, the Patriot’s Day Storm of 2007 will be long remembered for its meteorological significance and devastating power. Violent waves destroyed homes, businesses, coastal roads, and beaches, while forceful winds tore down power lines, leaving many residents in the dark for days. Portland had a peak wind of 59 mph and winds in Cape Elizabeth exceeded 80 mph measured on April 16th. An abnormally high spring tide plus a storm surge of 3 feet (2.72 feet at the Portland tide gauge) produced a high tide of 13.28 feet (the 7th highest tide measured since the early 1900’s).

As the storm deepened it stalled over the area for a full day before it slowly moved to the northeast. Very heavy rain fell on the coast with 5 to 8 inches over a 3-day period leading to river flooding.  In addition to the rain, strong winds caused significant storm surge and very large battering coastal waves. During this time there were four high tide cycles in which the water was near or above flood stage. Waves just off the coast were recorded at 25+ feet. This combination caused the tremendous amounts of damage seen during the storm. The flood resulted in peak stream flows with recurrence intervals greater than 100 years throughout most of York County, and recurrence intervals up to 50 years in Cumberland County.
 

Record Flooding on the St. John, Fish, and Aroostook Rivers, April-May 2008

The combination of snowmelt from record snowfall during the 2007-2008 winter season and between 1.5 and 5 inches of rain resulted in major and record flooding on the St. John and Fish Rivers from April 30th through May 1, 2008 . This flooding resulted in the closure of many roads and International Bridge, evacuated over 600 people, and flooded many roads, yards, and houses. The Fish River at Ft. Kent crested 13.93 feet on April 30th, the St. John River at Ft. Kent crested at 30.14 ft on April 30th, the St. Francis River near Conners, New Brunswick crested at 15.93 ft on May 1st, and the Aroostook River at Masardis crested at 18.33 ft on May 1st, which were all record flood levels.

Notable Flood: The 1976 "Groundhog Day Storm"

On February 2, 1976, downtown Bangor, Maine, was flooded with 12 feet (3.7 m) of water. The water surface elevation reached 17.46 feet (5.32 m) above the national geodetic vertical datum of 1929 (NGVD), approximately 10.5 feet (3.2 m) above the predicted astronomical tide at Bangor. Analysis of meteorological and hydrologic data indicates that the major cause of the flooding at Bangor was the combination of storm surge and high astronomical tide (storm tide). Anomalously high storm tide inundated the Penobscot River from Penobscot Bay and prevented the Kenduskeag from discharging into the Penobscot. Fresh water from Kenduskeag then overflowed directly into downtown Bangor. The storm surge generated on the open coast from Brunswick to Eastport and in the Penobscot Bay was funneled and amplified by hurricane-level south-southeasterly winds that “piled up” water into the Penobscot River to Bangor. The storm surge was generated by a fast-moving extratropical cyclone that had originated in the Gulf of Mexico three days before the event. The resulting flood was the third highest in Bangor since 1846 and is the first documented tidal flood at Bangor. Previously recorded floods at Bangor had been attributed to streamflow or backwater from debris or ice jams. 

Damages were estimated to be $2.6 million by the Maine Office of Civil Emergency Preparedness. No deaths were reported. Because the unusually high water in Bangor occurred suddenly, was of short duration, and involved a large volume of water, it was considered to be a "flash flood." The flood peak occurred late morning on February 2, 1976. Flood waters rose very quickly; it was estimated that it took less than 15 minutes for the water to reach maximum depth. Office workers could see the rising waters, but many could not get to their cars. Several people were caught by the flood as they tried to move their cars and had to be rescued. The flood submerged approximately 200 motor vehicles and many downtown businesses were inundated. Much of the damage was in flooded basements and in the cellar vaults of several downtown banks. There was a power loss in the area and electrical damage sparked at least two fires. Coastal areas from Brunswick to Eastport experienced substantial beach erosion and damage to coastal infrastructure. The storm surge reached a maximum height at Portland of 3.6 feet, Rockland 3.7 feet, and Bar Harbor 5.5 feet. Floodwaters began to recede an hour later. The following day, the rivers were well within their normal channels, but floodmarks remained visible and were used by the U.S. Geological Survey to document the extent of flooding.

Notable Flood: The 2021 "Halloween Storm"

On October 30-31, 2021 a rapidly developing area of low pressure tracked across western and southern Maine, delivering between 2-6.5 inches of rain within a matter of hours to various localities and driving extensive flash flooding and runoff. The annual probability of occurrence of this rainfall rate is 1 in 50, or 2%. Locations in Knox, Waldo, and York County experienced considerable damage to public infrastructure, as well as private homes and businesses, and the loss of electrical power to nearly 50,000 customers. Storm damage included culvert collapse and road washouts, flooding of a healthcare facility, and the most dramatic incident, the collapse of the Pepperell Mills Riverwalk along the Saco River in Biddeford. Damage estimates from flooding totaled $2.4 million.