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Mapping Flood Stages for Basin Communities

"The commission may study and determine the nature and extent of the flood plains of the Susquehanna River and its tributaries. Upon the basis of the studies, it may delineate areas subject to flooding, including but not limited to a classification of lands with reference to relative risk of flooding and the establishment of standards for flood plain use which will promote economic development and safeguard the public health, welfare, safety, and property."

--Article 6, Section 6.2 of the Susquehanna River Basin Compact, P.L. 91-575; 84 Stat. 1509 et seq.

What is a flood stage forecast map?

In the mid-1970s, the Susquehanna River Basin Commission (SRBC) developed a technique to produce flood stage forecast maps that reflect a range of stream stages that are tied to forecast points. SRBC prepared and widely distributed maps for 90 basin communities. Each community map covered all or portions of that community.

The maps provide details of flood plain areas and flood profiles with water level contours to show and explain what areas in a community will be inundated as flood waters reach certain levels (flood stages). These maps were developed using the flood profiles generated for the federal flood insurance program and relate to stream gages operated and maintained by the U.S. Geological Survey. Data from these stream gages are used by the National Weather Service as forecast points. Each map identifies the location of the stream gage used in preparing the map.

SRBC currently is seeking funds to produce maps for other communities.

Why are flood stage forecast maps important for the basin?

The Susquehanna River Basin is one of the nation's most flood prone areas--experiencing over six times the national average in damages per square mile each year. Of the 1,388 basin communities, 1,150 of those are affected by flooding. Furthermore, since the year 1810, the basin has been devastated by a major flood about every 20 years, with the most recent being the January 1996 flash flood.

Because the basin is so flood prone, it is important that community officials and residents understand what will happen to them when the basin's waterways reach flood stage. Many basin residents are unfamiliar with and confused by the term "flood stage." For example, when a flood stage of 20 feet is predicted, people want to know if that means the water will rise 20 feet from the river's bottom or 20 feet above normal levels. Does it mean a wall of water 20 feet high will come down the river or that the water will rise 20 feet over the flood plain?

The flood stage forecast maps convert forecasted flood stages, issued by the National Weather Service, to mean local water level elevations for communities within the map study area.

Who uses flood stage forecast maps?

The maps can be used by: 1) emergency personnel to identify areas that will be inundated first; 2) municipal officials to develop flood evacuation plans; and 3) individuals living near the flood plain to easily determine how the forecasted flood will affect them.

How long can the maps be used?

Once produced, the flood stage information is considered permanent. The only factor that may change is the addition of new development within the mapped area.

SRBC widely distributed the maps during the early 1980s to the community officials that were in office then. However, since there is a significant amount of turnover in local government, it is important that newly-elected officials be made aware of these maps so the communities can continue to benefit from them.

How are the flood stage forecast maps used?

Flood stage delineations, indicated by contour lines on the map, are placed usually in five (5) feet intervals. The intervals are chosen based on the topography of the flood plain. SRBC determined that a stage interval of five feet at the forecast point is generally adequate for interpretation and accuracy.

A flood stage forecast map is used after the National Weather Service or another reliable source predicts the expected flood stage for a specific event. Once that information is available, the map user can refer to the map and follow the instructions included on each map. In the next column is a sample instruction with the associated graphic.

The delineation lines placed on the map simply define the area of inundation to be expected if the forecasted flood stage happens to be one of the pre-selected stage interval lines. If the forecasted stage is not one that has been placed on the map, the map user must interpolate between stage interval lines. A more conservative approach, especially where delineations are provided at less than five feet intervals, is to use the next highest stage inundation line shown when considering the next course of action.

flood map

HOW TO USE THE MAP

  1. Locate your property on the map.
  2. Determine the flood stage which will affect you. Consider not only water on your property, but also water preventing your evacuation.
  3. Listen for forecasts on the radio and television of stages at the stream gage used on the map.
  4. If the forecasted stage could affect you, begin emergency actions. If not, listen for revised forecasts until waters recede.

EXAMPLE: If you live in House A, river stages greater than 24 feet [indicated as 24 on the map] would affect your property directly. If your evacuation route goes through a low-lying area, emergency action may be required at a lower stage than the stage affecting your property.

Available Flood Stage Forecast Maps

Maps for areas marked with an * are available from the U.S. Army Corps of Engineers.


Municipality County Coverage   Reach


Full or Partial
Pennsylvania


Allison Twp. Clinton P Jersey Shore-Lock Haven
Athens Boro. Bradford F Athens-Sayre-Susquehanna River and Chemung River
Athens Twp. Bradford F Athens-Sayre-Susquehanna River and Chemung River
Bald Eagle Twp. Clinton P Jersey Shore-Lock Haven
Bloomsburg Town Columbia F Town of Bloomsburg
Brady Twp. Lycoming F Turbot Twp. To Lycoming County
Buffalo Twp. Perry P Upper Paxton
Buffalo Twp. Union P Winfield to Lewisburg
Burnham Boro. Mifflin F Lewistown-Juniata/Kischocoquillas
Castania Twp. Clinton P Jersey Shore-Lock Haven
Chapman Twp. Clinton * *
Clinton Twp. Lycoming P Turbot Twp. To Lycoming County
Columbia Boro. Lancaster F Lower Susquehanna
Cunningham Twp. Luzerne P Shickshinny
Danville Boro. Montour F Danville
Delaware Twp. Northumberland F Turbot Twp. To Lycoming County
Derry Twp. Dauphin F Harrisburg-Swatara Cr. And Backwater
Derry Twp. Mifflin F Lewistown-Juniata/Kischocoquillas
Duboistown Boro. Lycoming * *
Duryea Boro. Luzerne F Wyoming Valley
East Buffalo Twp. Union F Winfield to Lewisburg
East Chillisquaque Twp. Northumberland P Winfield to Lewisburg
East Donegal Twp. Lancaster P Lower Susquehanna
East Hanover Twp. Dauphin F Harrisburg-Swatara Cr.
East Manchester Twp. York P York-Condorous Cr.
East Pennsboro Twp. Cumberland F Twp. Of East Pennsboro-River and Conodoquinet Cr.
Exeter Boro. Luzerne F Wyoming Valley
Fairfield Twp. Lycoming * *
Fairview Twp. York F Yellow Breeches
Flemington Boro. Clinton F Jersey Shore-Lock Haven
Forty Fort Boro. Luzerne F Wyoming Valley
Gamble Twp. Lycoming * *
Gregg Twp. Union F Turbot Twp. To Lycoming County
Harrisburg City Dauphin * *
Hellam Twp. York P Lower Susquehanna/Long Level Area-York County
Hepburn Twp. Lycoming * *
Hershey Village (Derry Twp.) Dauphin F Harrisburg-Swatara Cr.
Hughesville Boro. Lycoming * *
Hummelstown Boro. Dauphin F Harrisburg-Swatara Cr.
Jenkins Twp. Luzerne F Wyoming Valley
Jersey Shore Boro. Lycoming * *
Kelly Twp. Union F Winfield to Lewisburg/Turbot Twp. To Lycoming County
Kingston Boro. Luzerne P Wyoming Valley
Leidy Twp. Clinton * *
Lewis Twp. Lycoming * *
Lewisburg Boro. Union F Winfield to Lewisburg
Lewistown Boro. Mifflin F Lewistown-Juniata/Kischocoquillas
Lisburn Village (Lower Allen Twp.) Cumberland F Yellow Breeches
Liverpool Boro. Perry F Upper Paxton
Liverpool Twp. Perry F Upper Paxton
Lock Haven City Clinton * *
Londonderry Twp. Dauphin F Harrisburg-Swatara Cr. And Backwater
Long Level Area York F Long Level Area-York County
Lower Allen Twp. Cumberland F Yellow Breeches
Lower Augusta Twp. Northumberland P Sunbury to Selingsgrove
Lower Mahanoy Twp. Northumberland P Upper Paxton
Lower Swatara Twp. Dauphin F Harrisburg-Swatara Cr. And Backwater
Lower Windsor Twp. York P Long Level Area-York County
Loyalsock Twp. Lycoming * *
Lycoming Twp. Lycoming * *
Mahoning Twp. Montour P Danville
Manchester Twp. York F York-Condorous Cr.
Marietta Boro. Lancaster F Lower Susquehanna
Middletown Boro. Dauphin F Harrisburg-Swatara Cr. And Backwater
Mill Hall Boro. Clinton F Jersey Shore-Lock Haven
Millersburg Boro. Dauphin F Upper Paxton
Milton Boro. Lycoming * *
Milton Boro. Northumberland * *
Monoghan Twp. York F Yellow Breeches
Monroe Twp. Snyder F Sunbury to Selingsgrove
Montgomery Boro. Lycoming * *
Montoursville Boro. Lycoming * *
Muncy Boro. Lycoming * *
Muncy Creek Twp. Lycoming * *
New Cumberland Boro. Cumberland F Yellow Breeches
Newton Twp. Lackawanna F Wyoming Valley
North York Boro. York F York-Condorous Cr.
Noyes Twp. Clinton * *
Old Lycoming Twp. Lycoming * *
Penn Twp. Snyder P Sunbury to Selingsgrove
Piatt Twp. Lycoming * *
Pittston Boro. Luzerne F Wyoming Valley
Plains Twp. Luzerne F Wyoming Valley
Point Twp. Northumberland P Winfield to Lewisburg/Danville
Ransom Twp. Lackawanna F Wyoming Valley
Renovo Boro. Clinton * *
Riverside Boro. Northumberland F Danville
Royalton Boro. Dauphin F Harrisburg-Swatara Cr. And Backwater
Rush Twp. Northumberland P Danville
Sand Beach Village Dauphin F Harrisburg-Swatara Cr.
Sayre Boro. Bradford F Athens-Sayre-Susquehanna River and Chemung River
Selingsgrove Boro. Snyder F Sunbury to Selingsgrove
Shamokin Dam Boro. Snyder F Sunbury to Selingsgrove
Shickshinny Boro. Luzerne F Shickshinny
South Hanover Twp. Dauphin F Harrisburg-Swatara Cr.
South Renovo Boro. Clinton * *
South Waverly Boro. Bradford F Athens-Sayre-Susquehanna River and Chemung River
Spring Garden Twp. York P York-Condorous Cr.
Springettsbury Twp. York F York-Condorous Cr.
Sunbury City Northumberland F Sunbury to Selingsgrove
Swatara Twp. Dauphin F Harrisburg-Swatara Cr.
Swoyersville Boro. Luzerne F Wyoming Valley
Turbot Twp. Northumberland F Turbot Twp. To Lycoming County
Union Deposit Village Dauphin F Harrisburg-Swatara Cr.
Union Twp. Union P Winfield to Lewisburg
Upper Allen Twp. Cumberland F Yellow Breeches
Upper Augusta Twp. Northumberland P Sunbury to Selingsgrove
Upper Fairfield Twp. Lycoming * *
Upper Paxton Twp. Dauphin F Upper Paxton
Watsontown Boro. Northumberland F Turbot Twp. To Lycoming County
West Chillisquaque Twp. Northumberland F Winfield to Lewisburg/Turbot Twp. To Lycoming County
West Fairview Boro. Cumberland F Twp. Of East Pennsboro-River and Conodoquinet Cr.
West Hempfield Twp. Lancaster F Lower Susquehanna
West Manchester Twp. York P York-Condorous Cr.
West Pittston Boro. Luzerne F Wyoming Valley
West Wyoming Boro. Luzerne F Wyoming Valley
West York Boro. York F York-Condorous Cr.
White Deer Union F Turbot Twp. To Lycoming County
White Deer Twp. Union F Turbot Twp. To Lycoming County
Williamsport City Lycoming * *
Winfield Union F Winfield to Lewisburg
Wolf Twp. Lycoming * *
Wormleysburg Boro. Cumberland P Twp. Of East Pennsboro-River
Wrightsville Boro. York F Lower Susquehanna/Long Level Area-York County
Wyoming Boro. Luzerne F Wyoming Valley
York City York F York-Condorous Cr.




  Maryland


Havre de Grace City Harford F Conowingo Dam to Havre de Grace
Perryville Town Cecil F Conowingo Dam to Havre de Grace
Port Deposit District Cecil F Conowingo Dam to Havre de Grace
Port Deposit Town Cecil F Conowingo Dam to Havre de Grace
New York


Addison Town Steuben P Addison
Addison Village Steuben F Addison
Barker Town Broome * *
Bath Village Steuben * *
Big Flats Town Chemung F Big Flats
Big Flats Town Chemung * *
Binghamton City Broome F Binghamton-Susquehanna River and Chenago River
Campbell Town Steuben * *
Chemung Town Chemung P Athens-Sayre-Chemung River
Chenango Town Chenango * *
Cohocton Town Steuben * *
Conklin Town Broome * *
Dickinson Town Broome F Binghamton-Chenango River
Elmira City Chemung F Newtown Cr.
Elmira City Chemung * *
Elmira Heights Village Chemung F Newtown Cr.
Elmira Town Chemung F Newtown Cr.
Endicott Village Broome F Binghamton-Susquehanna River
Erwin Town Steuben * *
Fenton Town Broome * *
Greene Town Chenango * *
Horseheads Town Chemung F Newtown Cr.
Horseheads Village Chemung F Newtown Cr.
Johnson City Village Broome F Binghamton-Susquehanna River
Kirkwood Town Broome * *
Owego Village Tioga F Owego
Port Dickinson Village Broome F Binghamton-Chenango River
Southport Town Chemung * *
Union Town Broome F Binghamton-Susquehanna River
Vestal Town Broome F Binghamton-Susquehanna River