NOAA Technical Report NOS CO-OPS 0074

Gap Analysis of the Great Lakes 

Component of the National Water Level 

Observation Network (NWLON) 

Silver Spring, Maryland

September 2014 

noaa National Oceanic and Atmospheric Administration

U.S. DEPARTMENT OF COMMERCE

National Ocean Service

Center for Operational Oceanographic Products and Services
Center for Operational Oceanographic Products and Services

National Ocean Service

National Oceanic and Atmospheric Administration

U.S. Department of Commerce

The National Ocean Service (NOS) Center for Operational Oceanographic Products and Services 

(CO-OPS) provides the National infrastructure, science, and technical expertise to collect and 

distribute observations and predictions of water levels and currents to ensure safe, efficient and 

environmentally sound maritime commerce. The Center provides the set of water level and tidal 

current products required to support NOS’ Strategic Plan mission requirements, and to assist in 

providing operational oceanographic data/products required by NOAA’s other Strategic Plan 

themes. For example, CO-OPS provides data and products required by the National Weather 

Service to meet its flood and tsunami warning responsibilities. The Center manages the National 

Water Level Observation Network (NWLON), a national network of Physical Oceanographic 

Real-Time Systems (PORTS®) in major U. S. harbors, and the National Current Observation 

Program consisting of current surveys in near shore and coastal areas utilizing bottom mounted 

platforms, subsurface buoys, and horizontal mounted sensors. The Center: establishes standards 

for the collection and processing of water level and current data; collects and documents user 

requirements which serve as the foundation for all resulting program activities; designs new 

and/or improved oceanographic observing systems; designs software to improve CO-OPS’ data 

processing capabilities; maintains and operates oceanographic observing systems; performs 

operational data analysis/quality control; and produces/disseminates oceanographic products. 
NOAA Technical Report NOS CO-OPS 0074

Gap Analysis of the Great Lakes 

Component of the National Water Level 

Observation Network (NWLON) 

Stephen K. Gill 

September 2014

U.S.DEPARTMENT OF COMMERCE

Penny Pritzker, Secretary

National Oceanic and Atmospheric Administration 

Dr. Kathryn Sullivan

NOAA Administrator

National Ocean Service

Dr. Holly Bamford, Assistant Administrator

Center for Operational Oceanographic Products and Services 

Richard Edwing, Director
NOTICE

Mention of a commercial company or product does not constitute an 

endorsement by NOAA. Use of information from this publication for 

publicity or advertising purposes concerning proprietary products or the 

tests of such products is not authorized.
iii 

TABLE OF CONTENTS 

TABLE OF CONTENTS ..................................................................................................... iii

LIST OF FIGURES .............................................................................................................. iv

LIST OF TABLES ................................................................................................................ vi 

EXECUTIVR SUMMARY ................................................................................................ vii

1.0 PURPOSE ..................................................................................................................... 1

2.0 BACKGROUND ........................................................................................................... 3

3.0 GAP ANALYSIS APPROACH .................................................................................. 19

4.0 INDIVIDUAL LAKE ASSESSMENTS ..................................................................... 29

5.0 SUMMARY OF FINDINGS AND RECOMMENDATIONS ................................... 65

ACKNOWLEDGEMENTS ................................................................................................. 69

REFERENCES ..................................................................................................................... 71

APPENDIX 1. Hourly Water Levels for each Great Lake from Calendar Year 2005 ............. 75

LIST OF ACRONYMS AND ABBREVIATIONS ............................................................. 81
iv 

LIST OF FIGURES 

Figure 1. Schematic drawing of the Great Lakes Hydrologic Water System. ................................. 3

Figure 2. Contoured rates of vertical velocities (in cm/century) derived from water level gauges 

surrounded by ICE-3G-model-derived velocities in the Great Lakes region (Mainville 

and Craymer 2005). ....................................................................................................... 6

Figure 3. Relative vertical velocities for each Great Lake (Mainville and Craymer, 2005). .......... 7

Figure 4. Individual station biases computed from trends analyses.(Mainville and Craymer, 

2005). ............................................................................................................................. 7

Figure 5. Comparison of long-term hydrographs of each Great Lake with long-term sea level 

variations at New York City (from GLERL, 2013)....................................................... 9

Figure 6a. USACE water level bulletin for Lake Superior for November 2013. Legend depicted is 

common to all following bulletin plots........................................................................ 11

Figure 7. Map of values of the Hydraulic Correctors (meters) for U.S. and Canadian water level 

stations. ........................................................................................................................ 16

Figure 8. IGLD 85 Heights (m) of LWD for the Great Lakes – St. Lawrence River System. ...... 17

Figure 9. Comparison of daily mean water levels for June 1985 and June 2013 in Lake Superior.

 ..................................................................................................................................... 22

Figure 10. Hourly height differences between Point Iroquois and Gros Cap, 2005-2011. ........... 24

Figure 11. X-Y plot scatterplot comparison of hourly heights for Gros Cap and Point Iroquois. 24

Figure 12. Hourly height differences between Harbor Beach and Essexville, 2005-2011. .......... 25

Figure 13. X–Y scatterplot comparison of hourly heights at Harbor Beach and Essexville, 2006-

2010. ............................................................................................................................ 26

Figure 14. U.S. and Canadian Water level Station locations for Lake Superior. .......................... 29

Figure 15. Daily mean water levels for Lake Superior for June 2013........................................... 30

Figure 16. Lake Superior correlation coefficients from Zervas (1997). ........................................ 31

Figure 17. Interpolated HCs for Lake Superior. ............................................................................ 32

Figure 18. Relative vertical velocities (in cm/century) for Lake Superior (Mainville and Craymer 

[2005]) relative to the lake outlet. ............................................................................... 33

Figure 19. Relationship of LWD and NAD 83 for Lake Superior. ............................................... 33

Figure 20. U.S. water level station locations for Lake Michigan and vicinity. ............................. 34

Figure 21. Lake Michigan correlation coefficients from Zervas (1997). ...................................... 36

Figure 22. Simultaneous plot of data from Green Bay, Menominee and Sturgeon Bay for May 

and June 2013. ............................................................................................................. 37

Figure 23. Bathymetric contours for Green Bay, Lake Michigan (from: 

http://www.ngdc.noaa.gov/mgg/greatlakes/lakemich_cdrom/html/area1.htm). ......... 38

Figure 24. Interpolated Hydraulic Correctors (in meters) for Lake Michigan. ............................. 39

Figure 25. Relative vertical velocities (in cm/century) for Lake Michigan and Lake Huron 

(Mainville and Craymer [2005]) relative to the lake outlet. ........................................ 39

Figure 26. Relationship of LWD to NAD83 for Lake Michigan. ................................................. 40

Figure 27. U.S. and Canadian water level station locations for Lake Huron and vicinity. ........... 41
v 

Figure 28. Lake Michigan/Lake Huron Daily Mean Water Levels June 2013. ............................ 42

Figure 29. Lake Huron correlation coefficients from Zervas (1997). The new U.S. station at 

Alpena has replaced Harrisville subsequent to his study. ........................................... 43

Figure 30. Water level variations for a sample period of May and June 2013 for Essexville and 

Harbor Beach. .............................................................................................................. 44

Figure 31. Bathymetric contour plots for Saginaw Bay, Lake Huron (from 

http://www.michigan.gov/documents/deq/deq-whm-hwp-dow-9-2007-Draft-CCRSRB-Fig-2-7_210883_7.pdf). ...................................................................................... 45

Figure 32. Interpolated Hydraulic Correctors for Lake Huron. ..................................................... 46

Figure 33. Vertical velocities (in cm/century) for Lake Michigan and Lake Huron (Mainville and 

Craymer [2005]) relative to the lake outlet. ................................................................ 46

Figure 34. Relationship of LWD to NAD83 for Lake Huron. ...................................................... 47

Figure 35. U.S. and Canadian Water level station locations for Lake St. Clair and vicinity. ....... 48

Figure 36. Lake St. Clair daily mean water levels for June 2013. ................................................. 49

Figure 37. U.S. and Canadian water level station locations for Lake Erie and vicinity. ............... 50

Figure 38. Lake Erie daily mean water Levels June 2013............................................................. 51

Figure 39. Lake Erie correlation coefficients from Zervas (1997). ............................................... 52

Figure 40. Interpolated Hydraulic Correctors for Lake Erie. ........................................................ 53

Figure 41. Vertical velocities (in cm/century) for Lake Erie (Mainville and Craymer [2005]) relative 

to the lake outlet. ........................................................................................................... 54

Figure 42. Relationship of LWD to NAD83 for Lake Erie. .......................................................... 54

Figure 43. U.S. and Canadian water level Station locations for Lake Ontario. ............................ 55

Figure 44. Lake Ontario Daily Mean Water Levels June 2013. .................................................... 56

Figure 45. Lake Ontario correlation coefficients from Zervas (1997). ......................................... 57

Figure 46. Interpolated Hydraulic Correctors for Lake Ontario. ................................................... 58

Figure 47. Vertical velocities (in cm/century) for Lake Ontario (Mainville and Craymer [2005]) 

relative to the lake outlet. .............................................................................................. 58

Figure 48. Relationship of LWD to NAD83 for Lake Ontario ..................................................... 59

Figure 49. Location of NWLON, Canadian, and USACE water level stations on St. Marys River. 

Note: The station at Lookout #4 has been replaced by a nearby station at West 

Neebish Island. Little Rapids is now a NOAA-operated station (http://www.greatlakes.net/envt/water/levels/levels_current.html). ........................................................ 60

Figure 50. Locations of U.S. NWLON (blue), Canadian (red), and USACE (gray) water level 

stations on the St. Clair River (http://www.greatlakes.net/envt/water/levels/levels_current.html). ........................................................ 61

Figure 51. Locations of U.S. (blue), Canadian (red), and USACE (gray) water level stations on 

the Detroit River (http://www.great-lakes.net/envt/water/levels/levels_current.html).

 ..................................................................................................................................... 62

Figure 52. Location of U.S. Stations on the Niagara River 

(http://tidesandcurrents.noaa.gov/map/). ..................................................................... 63 
vi 

Figure 53. Location of NWLON and Canadian Stations in the international section of the St. 

Lawrence River. Note: Stations at Kingston and Cape Vincent are on Lake Ontario 

and are included in the analyses for that Lake. Power entity and Seaway Authority 

Stations are not included (http://www.greatlakes.net/envt/water/levels/levels_current.html) ......................................................... 64

Figure 54. Relationship of projected elevation differences from vertical velocities to calculated 

mean elevation differences 2005 – 2011 for station pairs in the five major lakes. ..... 67

LIST OF TABLES 

Table 1. Comparison statistics for Lake Superior station pairs using seven years of hourly height 

data from 2005-2011. Elevations were obtained from U.S. and Canadian databases relative 

to IGLD 1985 ( HC applied). Projected mean differences are in italics. ............................. 31

Table 2. Comparison statistics for Lake Michigan station pairs using seven years of hourly height 

data from 2005-2011. Elevations were obtained from U.S. and Canadian databases 

relative to IGLD 1985 ( HCs applied). Projected mean differences are in italics. .............. 35

Table 3. Comparison statistics for Lake Huron station pairs using seven years of hourly height data 

from 2005-2011. Elevations were obtained from U.S. and Canadian databases relative to 

IGLD 1985 (HCs applied). Projected mean differences are in italics. ................................ 42

Table 4. Comparison statistics for Lake St. Clair station pairs using seven years of hourly height 

data from 2005-2011. Elevations were obtained from U.S. and Canadian databases 

relative to IGLD 1985 ( HCs required for Belle River only). ............................................. 49

Table 5. Comparison statistics for Lake Erie station pairs using seven years of hourly height data 

from 2005-2011. Elevations were obtained from U.S. and Canadian databases relative to 

IGLD 1985 (HCs applied). Projected mean differences are in italics. ................................ 52

Table 6. Comparison statistics for Lake Ontario station pairs using seven years of hourly height data 

from 2005-2011. Elevations were obtained from U.S. and Canadian databases relative to 

IGLD 1985 ( HCs applied). Projected mean differences are in italics. ............................... 57

Table 7. Comparison statistics for St. Marys River station pairs using seven years of hourly height 

data from 2005-2011. Elevations were obtained from databases relative to IGLD 1985. ... 60

Table 8. Comparison statistics for St. Clair River station pairs using seven years of hourly height 

data from 2005-2011. Elevations were obtained from database relative to IGLD 1985. .... 61

Table 9. Comparison statistics for Detroit River station pairs using seven years of hourly height 

data from 2005-2011. Elevations were obtained from database relative to IGLD 1985. .... 62

Table 10. Comparison statistics for the Niagara River station pairs using seven years of hourly height 

data from 2005-2011. Elevations were obtained from database relative to IGLD 1985. .... 63

Table 11. Comparison statistics for the St. Lawrence River station pairs using seven years of hourly 

height data from 2005-2011. Elevations were obtained from database relative to IGLD 

1985 ..................................................................................................................................... 64
vii 

Gap Analysis of the Great Lakes Component of the NWLON 

EXECUTIVE SUMMARY 

This gap analysis for the Great Lakes component of the National Oceanic and Atmospheric 

Administration (NOAA) National Water Level Observation Network (NWLON) is a companion 

to an earlier NOAA gap analysis of the tide station network prepared in 2008 and updated in 

2014 (A Network Gaps Analysis for the National Water Level Observation Network, NOAA 

Technical Memorandum NOS CO-OPS 0048). Each report looks at the fundamental 

requirements for networks of long-term continuous water level stations and assesses the present 

day network configuration and how well it meets requirements. For the Great Lakes NWLON, 

the requirements are inherently bi-national in nature, as water level monitoring and regulation is 

a dual responsibility of both Canada and the United States. The international agreements and 

coordinating bodies all have specific needs for water level measurement, and they are briefly 

discussed in this document to provide a sense of the complexity of the Great Lakes hydrological 

system. The NOAA NWLON covers the U.S. shoreline and a similar Canadian network covers 

the Canadian shoreline. 

The gap analysis presented here looks at data from both networks in a comprehensive and an 

integrated manner. The lake-by-lake assessments draw upon previous studies, reports, and 

analyses as well as a statistical comparative analysis of a recent seven-year period of data 

(2005-2011). The impact of differences in the rates of vertical land movement across the Great 

Lakes emerges as one of the most important parameters in defining the water level station 

network configuration requirements. The present network of 53 NWLON stations has very little 

redundancy in spatial coverage, and several areas with weaknesses in coverage are shown. Three 

(3) specific locations have been identified as NWLON gaps; two in Lake Michigan and one in 

Lake St. Clair. This report will provide input into program planning for the upcoming update of 

the International Great Lakes Datum from IGLD 1985 to IGLD 2020. 

1 

Gap Analysis of the Great Lakes Component of the NWLON

1.0 PURPOSE 

A network gap analysis for the National Water Level Observation Network (NWLON) was 

completed in 2008 (Gill and Fisher, 2008). However, the Great Lakes component of the 

NWLON was not included in that analysis because an entirely different evaluation methodology 

needed to be developed for the Great Lakes. The purpose of this document is to provide a gap 

analysis for the Great Lakes component of the NWLON. This assessment is necessary to ensure 

that observing system goals are being met by the operational networks, the current and projected 

resources are aligned to meet existing and future requirements, the data meet the needs of other 

federal agencies who rely upon NOAA data to meet their missions, and the future and emerging 

requirements can contribute to NOAA/Center for Operational Oceanographic Products and 

Services (CO-OPS) strategic planning. The NWLON is a managed network of permanent 

stations established to obtain sustained long-term high quality water level measurements. 

This report primarily also draws upon information compiled for a series of Great Lakes Water 

Level, Hydraulic Corrector, and Geodetic Datum Assessment reports that are presently being 

completed by CO-OPS for each lake (CO-OPS 2013). One of the primary purposes of these 

individual reports is to define the requirements for the seasonal gauge program effort associated 

with the 2020 update to the International Great Lakes Datum (IGLD). This report reviews the 

NWLON station requirements to assess the network against international and national 

requirements for datum control and continuous monitoring for a variety of applications. 

By design, this report contains a substantial background section that has two main purposes: first, 

to provide those with little knowledge of the Great Lakes a basic understanding of Great Lakes 

hydrology, lake level variations, and the human interaction with management and regulation; and 

second, to provide background information on lake level variations such that the reader can 

better appreciate the approach to identifying gaps and how filling those gaps would improve the 

water level network.

While focusing on the U.S. network perspective, it is understood that ultimately the goal for 

observing systems in the Great Lakes is bi-national, requiring close collaboration and 

coordination with observing system agency counterparts in both countries. It is hoped that this 

report can be used as constructive input into future coordinated assessments for the entire Great 

Lakes observing system.

3 

2.0 BACKGROUND

The Great Lakes-St. Lawrence River System is a large, complex natural hydrologic system of 

lakes and rivers that has been altered over time by dredging, locks, dams, hydroelectric power 

facilities, canals, and diversions. The Great Lakes Basin is approximately 295,000 square miles 

in size, covering both Canadian and U.S. territory, and includes eight U.S. states and two 

Canadian provinces. The nature of the complexity in the system is illustrated in Figure 1. The 

complexity of natural and human-influenced variability of the lake levels and inter-connecting 

flows in this system presents a continual challenge for the bi-lateral interests of both countries. 

The water balance of the system includes inflow components to each lake in the following forms: 

over-lake precipitation, runoff, ground water seepage, diversions into a particular lake, and flow 

from the connecting channels. Outflow from any particular lake includes components from 

evaporation, flow into a connecting channel, diversions away from a particular lake, and 

consumptive use (Neff and Nicholas, 2005). The foundational requirement for the Great Lakes 

NWLON is vertical control for safe navigation and for water resource management in the lakes. 

Two of the most important drivers for international management of the water levels are safe and 

efficient navigation for the port facilities and maintaining adequate flows for the hydroelectric 

power plants. Other needs include water management, flood damage reduction, water supply, 

and recreation. 

Figure 1. Schematic drawing of the Great Lakes Hydrologic Water System. 
4 

Gronewold et al. (2013) note the increased emphasis on the inclusion of climate change and 

adaptive management by the Great Lakes management community, which further justifies the 

requirement for a robust monitoring infrastructure and strong research program.

2.1 Regulation and Water Management

The International Joint Commission (IJC) and their Boards of Control provide for the 

international regulatory framework for the Great Lakes. The outflows from Lakes Superior and 

Ontario are regulated under the auspices of the IJC. A separate bi-national Coordinating 

Committee on Great Lakes Basic Hydraulic and Hydrologic Data, which is not under the 

auspices of the IJC, performs applicable hydrologic and hydraulic studies, coordinates data 

collection activities, coordinates data for use by both countries, sets protocols, performs studies, 

and establishes the vertical reference control datum. 

Lake Superior 

The Long Lac and Ogoki diversions re-direct water from Canadian rivers into Lake Superior 

(approximately 5,580ft 3 /s). Outflows into Lakes Michigan and Huron from the St. Marys River 

are affected by hydroelectric power plants, the Soo Navigation Locks, the International 

Compensating Works dam, and municipal and industrial uses. The IJC’s International Lake 

Superior Board of Control established the basic objectives for and limits to the regulation of 

Lake Superior's outflow and established the Lake Superior Board of Control to determine 

outflows and oversee operation of the various control works. The regulation plan acknowledges 

the needs of various interests on Lake Superior, the St. Marys River, and Lake Michigan-Huron, 

including navigation, hydropower, fishery, and riparian rights. The main objective of the present 

regulation plan is to determine a flow that will bring the levels of Lakes Superior, Huron, and 

Michigan to the same relative position within their respective ranges of historic seasonal 

averages. Average monthly outflows are 75,000 ft3

/s (Neff and Nicholas, 2005). 

Lake Michigan and Lake Huron 

The Straits of Mackinac allow Lakes Michigan and Huron to be considered one lake for 

hydraulic flow and lake level applications. The net flow of this system is from Lake Michigan 

into Lake Huron; however, there is evidence of a strong oscillatory flow pattern (Anderson and 

Schwab, 2013). The Lake Michigan Diversion at Chicago accounts for an average outflow of 

3,200 ft3

/s to the Chicago Sanitary and Ship Canal System. Natural outflow occurs through the 

St. Clair River, Lake St. Clair, Detroit River connecting waterway, and into Lake Erie. The 

connecting waterway has no man-made water level maintenance mechanism or diversion; it does 

have a navigation channel which is maintained by the U. S. Army Corps of Engineers (USACE) 

Detroit District office and the Canadian Coast Guard. This waterway accounts for a monthly 

average outflow of 188,000 ft3/s from Lakes Huron and Michigan (Neff and Nicholas, 2005). 

Historically, deepening the navigation channel was found to have a significant effect in the levels 

of Lakes Michigan and Huron. The flows in this waterway have been the subject of recent 

intense investigation given the recent low lake levels (IUGLSB, 2009). 

Lake Erie 

Lake Erie is particularly susceptible to high frequency seiche events in which significant 

differentials in water level elevation (greater than 1 meter) can occur between the western and 
5 

eastern ends of the lake. When high lake levels occur on the western end of the lake, the inflow 

from the Detroit River can be impeded and, on rare occasion, temporarily reversed. In the other 

direction (west to east) during significant wind events, the western end of Lake Erie can drop 

significantly, leaving commercial ships transiting into Lake Erie from the Detroit River without 

adequate draft to do so. The Niagara River and the Welland Canal are the main conduits of 

outflow from Lake Erie into Lake Ontario. 

The amount a power company can divert from the Niagara River is defined by the required flow 

over Niagara Falls as specified in the 1950 Treaty between Canada and the United States of 

America concerning uses of the water of the Niagara River. The average monthly outflow into 

Lake Ontario from Lake Erie is approximately 210,000 ft3/s (Neff and Nicholas, 2005). 

Lake Ontario – St. Lawrence River 

One requirement of the IJC’s International St. Lawrence River Board of Control is to regulate 

Lake Ontario within a monthly-mean target range from 74.2 to 75.4 meters (243.3 to 247.3 feet) 

above sea level (presently defined by IGLD 1985). The International St. Lawrence River Board 

of Control was established by IJC in a 1952 order to ensure outflows from Lake Ontario meet the 

requirements of the Commission's Order of Approval. The Board implements the current 

regulation plans and conducts studies as requested by the Commission in order to improve its 

regulatory practices. Water flows from Lake Ontario into the St. Lawrence River are regulated 

by several power entity control structures, dams, and locks. The average monthly outflow from 

Lake Ontario into the St. Lawrence River is 246,000 ft3

/s (Neff and Nicholas, 2005). 

2.2 Crustal Movement 

Current crustal movement in the Great Lakes basin is the result of the natural rebound of the 

Earth’s crust following the removal of the weight of the glaciers that covered the region 

thousands of years ago. The force pushing down on the Earth’s crust was much less near the 

edges of the glacial shield. The Earth’s crust north of the Great Lakes in Canada was pressed 

down by as much as 3 km of ice in some areas during the last glacial era. When the ice began 

melting some 10,000 years ago, the crust started rebounding. This phenomenon is historically 

known as Post-Glacial Rebound (PGR) and is the key reason why the Great Lakes vertical 

control datum requires updating approximately every thirty years. The term PGR is gradually 

being replaced by the term Glacial Isostatic Adjustment (GIA). This is in recognition that the 

response of the Earth to glacial loading and unloading is not limited to the upward rebound 

movement but also involves downward land movement, horizontal crustal motion, changes in 

global sea levels, the Earth's gravity field, induced earthquakes, and changes in the rotational 

motion. 

As a result of GIA, there are significant differences in rates of vertical movement of the earth’s 

crust across the Great Lakes-St. Lawrence River System (Mainville and Craymer, 2005; Lee and 

Southam, 1994). In general, the land north of the Great Lakes is rising and the land south of the 

Great Lakes, never covered by the glaciers, is subsiding (see Figure 2). As seen in Figure 2, the 

“0” axis from positive to negative vertical land movement transects the lakes, and regional 

movement is on the order of several centimeters per century. The long-term time series of 
6 

monthly mean water levels from the U.S. and Canadian networks were used to construct the 

model for vertical land motion. 

Figure 2. Contoured rates of vertical velocities (in cm/century) derived from water level gauges surrounded by 

ICE-3G-model-derived velocities in the Great Lakes region (Mainville and Craymer 2005). 

This GIA impact on datum control is somewhat analogous to the requirement in the U.S. to 

periodically update the National Tidal Datum Reference System to new 19-year periods due to 

relative sea-level change (CO-OPS, 2001; Zervas, et al., 2013).The crustal movement has a 

significant effect on the vertical datum planes used to reference water levels and also changes the 

hydraulic flow characteristics of the connecting channels. To be meaningful, water level datum 

elevations relative to the land must be updated over time. 

The effects of GIA have recently been studied further as part of the International Upper Great 

Lakes Study (Bruxer and Southam, 2008; IUGLS, 2009). The study reports that the physical and 

apparent effects of GIA result from the relative differences in the movement of the earth’s crust 

and the impact this has on water levels observed at different locations around each of the lakes 

(i.e., the apparent observed surface elevation at any given time and location relative to the lakewide mean levele). The study found that the apparent effect of GIA accounts for about 4 to 5 cm 

(1.6 to 2 in) of the approximately 23 cm (9 in) decline in the head difference between Lake 

Michigan-Huron and Lake Erie between 1963 and 2006, which was determined based on water 

levels recorded at Harbor Beach, Michigan on Lake Huron and Cleveland, Ohio on Lake Erie. 

For this gap analysis, the differential rates of vertical movement relative to each lake’s outlet is 

most relevant, as these rates have significant impact on apparent water levels observed at 

different locations around each lake over time (Bruxer and Southam, 2008). As shown in 
7 

Figure 3, the relative vertical velocities can be determined relative to the stations closest to the 

outlet for each lake (Mainville and Craymer, 2005). 

Figure 3. Relative vertical velocities for each Great Lake (Mainville and Craymer, 2005). 

The individual station site biases (Figure 4) determined from the trend analyses are values 

analogous to the hydraulic corrector values (discussed later in the Section 2.8) and demonstrate 

the actual variability of the lake surface from lake-wide mean values. 

Figure 4. Individual station biases computed from trends analyses.(Mainville and Craymer, 2005). 
8 

The geospatially varying rates have implications on the need to maintain a properly configured 

network of stations with repeat GPS surveys or co-located Continually Operating Reference 

Stations (CORS) for both monitoring the changes in hydraulic characteristics of the lake basins 

and the ongoing need to update the reference datum within the system. More precise estimates of 

the vertical land movement should be possible through continued establishment of CORS. Over 

time, precise independent estimates of vertical land motion will be possible. Snay et al. (2007) 

estimate that generally at least twelve years of data will be required before the standard errors of 

the vertical velocity estimates will be sufficiently small for the velocities to be used with 

confidence. 

2.3 Long-term Average Water Levels 

Using subsets of Canadian and U.S. station networks, annual and monthly average water levels 

have been carefully compiled from observations back as far as 1860 allowing for assessments of 

the long-term changes in water levels in each lake (GLERL, 2014). For overall context on longperiod variability, Figure 5 illustrates the use of multiple water level stations to compile lakewide averaging to produce a hydrograph for each lake, showing the monthly and annual time 

series since 1860.The graphic puts the lake level variations into long-term context with sea level 

variations over the same time period at the Battery, New York City. The historical 

maximum/minimum range of lake levels over this time period are a little over 1.0 m for Lake 

Superior, 1.5 m for Lakes Michigan, Huron and Erie, and 1.6 m for Lake Ontario. The relatively 

lower lake levels since 2000 for the Lakes Superior, Michigan, and Huron and the regulation of 

Lake Superior (since 1916) and Lake Ontario (since 1960) are particularly noticeable in Figure 5. 

Gronewold et al. (2013) describes the long-term variability of Great Lakes water levels with the 

perspective of long-term drivers. Major drivers include over-lake precipitation, over-lake 

evaporation, and land surface runoff. Regional climate changes propagate changes to the water 

budget and the water levels. Updated hydrographs and water level dashboard products can be 

obtained at: http://www.glerl.noaa.gov/data/now/wlevels/levels.html. 

The stations used in lake level calculations have changed over time based on station operation 

and data availability, and the official international coordinated period of record presently used is 

1918-2012. This official period is after the implementation of regulation of Lake Superior and 

includes documented adjustments for GIA. 
9 

Figure 5. Comparison of long-term hydrographs of each Great Lake with long-term sea level variations at New 

York City (from GLERL, 2013). 

2.4 Monthly Mean Water Levels 

Both the U.S. and Canadian water level networks contribute to important operational data 

products and forecasts. Monthly mean values for each station are compiled, and a coordinated 

subset of Canadian and U.S. gauges is used to calculate the lake-wide average monthly water 

levels for each month. These values are put into historical perspective in a Monthly Bulletin of 

Lake Levels product compiled and disseminated by the United States Army Corps of Engineers 

(USACE, 2014) and Canada. The station locations used to compute the lake-wide means were 

selected based on their data quality, period of record, and geography. 

Figures 6a-e are example USACE bulletin graphics for each lake showing the recent monthly 

means, the historical long-term average monthly means, the historic maxima and minima 

monthly means, and the 6-month forecast, all relative to Low Water Datum (LWD). Bulletins are 

available at: 
10 

http://www.lre.usace.army.mil/Missions/GreatLakesInformation/GreatLakesWaterLevels/Water

LevelForecast/MonthlyBulletinofGreatLakesWaterLevels.aspx The Canadian Hydrographic 

Service puts out a similar bulletin using the same set of stations 

(http://www.waterlevels.gc.ca/C&A/bulletin.pdf). 

The coordinated network of stations presently used by agencies in both countries to calculate the 

monthly mean lake levels are as follows: 

Lake Superior

Duluth, Minn.; Marquette and Point Iroquois, Mich.; Michipicoten and Thunder Bay, 

Ontario 

Lake Michigan-Huron 

Harbor Beach, Mackinaw City and Ludington, Mich.; Milwaukee, Wis.; Thessalon and 

Tobermory, Ontario 

Lake St. Clair 

St. Clair Shores, Mich. and Belle River, Ontario 

Lake Erie 

Toledo and Fairport, Ohio (Note: replaced by Cleveland, Ohio in 2009); Port Stanley and 

Port Colborne, Ontario 

Lake Ontario 

Rochester and Oswego, N.Y.; Port Weller, Toronto, Cobourg and Kingston, Ontario 

Monthly mean water levels in the Great Lakes have a very strong annual variation. The annual 

range in lake levels is approximately 0.30 m for Lake Superior, 0.30 m for Lakes MichiganHuron, 0.40 m for Lake St. Clair, 0.35 m for Lake Erie, and 0.50 m for Lake Ontario. Maximum 

and minimum monthly means do not all occur during the same months for each lake. Figures 

6a-e show the uniqueness of the variations for each lake. Typically, the maxima/minima for 

Lake Superior occur August/March; Lakes Michigan-Huron occur July/February; Lake St. Clair 

occur July/February; Lake Erie occur June/December; and Lake Ontario occur June/December. 
11 

Figure 6a. USACE water level bulletin for Lake Superior for November 2013. Legend depicted is common to all 

following bulletin plots. 

Figure 6b. USACE water level bulletin for Lakes Michigan-Huron for November 2013. 
12 

Figure 6c. USACE water level bulletin for Lake St Clair for November 2013. 

Figure 6d. USACE water level bulletin for Lake Erie for November 2013. 
13 

Figure 6e. USACE water level bulletin for Lake Ontario for November 2013. 

The figures above show that lake levels in Lake Superior and Lake Michigan/Huron have 

recently been near or below their respective Chart Datum (LWD) elevations for large stretches of 

time. These low water stages have been of increasing concern to the coastal communities. Data 

from the existing gauge networks will provide the basis for any needed update of LWD 

elevations if the low lake levels do persist. 

2.5 Daily Mean Water Levels 

Using the same set of stations to establish the lake-wide values for the monthly bulletin 

described above, agencies in both countries compile and disseminate individual station daily 

means and lake-wide daily means (for example, from the USACE, see: 

http://w3.lre.usace.army.mil/hh/GreatLakesWaterLevels/GLWL-1MonthAgo-Meters.pdf). 

The daily mean values are used to monitor Great Lakes conditions for various purposes. This 

includes the determination of net basin supplies, which can be estimated as a residual of inflows, 

outflows, and change in storage (water level) of the lake over time. The variations in the daily 

means are also used to study lake dynamics and response to short-to-medium-term weather 

patterns. Lake surfaces are not uniformly level above a reference datum at all places at all times 

due to short-period variability in response to meteorological conditions. The effect of wind and 

differences in barometric pressure over the lake surface create imbalances in water level at 

various locations or various time scales. 

2.6 International Great Lakes Datum (IGLD) Control 

The vertical elevation datum for the Great Lakes, known as the International Great Lakes Datum 

(IGLD), first established in 1955 and last updated in 1985, needs to be updated every 25 to 30 

years to reflect continuous and differential changes in land surface elevations across the region. 

Earlier U.S. datum systems were established in 1903 and 1935. 
14 

The general adjustment of the North American Vertical Datum of 1988 (NAVD 88), a minimumconstraint adjustment of Canadian-Mexican-U.S. leveling observations, was performed holding 

fixed the height of the primary tidal bench mark, referenced to the new International Great Lakes 

Datum of 1985 (IGLD 85) local mean sea level height value at Father Point/Rimouski, Quebec, 

Canada. Father Point/Rimouski is an IGLD water-level station located at the mouth of the St. 

Lawrence River and is the reference station used for IGLD 85. The only difference between 

IGLD 85 and NAVD 88 is that IGLD 85 bench mark values are given in dynamic height units, 

and NAVD 88 values are given in Helmert orthometric height units. The geopotential numbers 

of bench marks are the same in both systems (Zilkoski et al., 1992). The use of dynamic heights 

provides an accurate measurement of potential hydraulic head, a requirement for the water 

management of the Great Lakes. 

IGLD 85 development involved a comprehensive re-leveling of bench mark networks around 

each lake. Leveling is no longer viable logistically or financially. A new method of updating 

dynamic heights, without re-leveling, must be implemented for development IGLD 2020 and is 

under discussion in the Coordinating Committee. In the U.S., the national vertical datum (NAVD 

88) is also being updated, also without re-leveling the country. Canada adopted a new vertical

datum (CGVD 2013) in November 2013. Access to the future vertical datum will be through the 

determination of an ellipsoid height (via the Global Navigation Satellite System or GNSS) minus 

a geoid undulation (from gravity data, including a new, nationwide airborne gravity campaign 

called GRAV-D) to determine orthometric heights. 

For IGLD, it is proposed that the determination of a dynamic height will begin with a GPSderived ellipsoid height at a location using a geoid model to obtain an orthometric height. From 

there, surface and airborne gravity will be used to determine a geopotential number. Normal 

gravity will then be used to determine dynamic heights. It is noted that the 1985 update from 

1955 did not include all stations where IGLD 55 was established. For instance, the seasonal 

gauging program was not part of that update. This has presented ongoing problems in being able 

to update datums for IGLD 2020 and in properly interpolating between stations. NOAA has been 

assessing these requirements for each in lake in a series of lake-by-lake assessment reports 

(CO-OPS 2013), and those recommendations will be included in planning for IGLD 2020. 

2.7 Hydraulic Correctors 

The mean water surfaces of the Great Lakes are considered to be geopotentially equal within 

each lake basin. Therefore, on any particular lake, at the time a new vertical datum is established, 

all Mean Water Level (MWL) values for gauging stations around a particular lake should 

coincide. The difference between NAVD 88 Dynamic Heights used to reference IGLD 85 is the 

application of a Hydraulic Corrector (HC) derived during the measurement period, 1982-1988, at 

each subordinate lake station. Only within each lake are Hydraulic Correctors used and applied 

at each of the lakes subordinate water level gauges to reference the base elevation, Low Water 

Datum/Chart Datum. In the sloping surfaces, rivers and channels, Dynamic Heights equal IGLD 

85 and HCs are not applied. 

For present definition, the MWL is the average water surface elevation for the summer months 

(June-September) for the years 1982-1988 referenced to each gauging station’s Primary Bench 

Mark (PBM) Dynamic Height. The MWL at each gauging station was then treated as a bench 
15 

mark in the network adjustment. Following the adjustment it was found that the MWL at each 

gauging station on a particular lake was slightly different from the expected value if the lake is a 

geopotential surface. The differences were due to cumulative differences in the leveling 

adjustments, steric effects, and dynamic effects. The Coordinating Committee (1995) decided to 

apply the Hydraulic Corrector to each subordinate gauge station on each lake so that each station 

has the same dynamic elevation as the Master Station for that lake. This is accomplished by 

holding the Master Station as the controlling value and comparing all other gauging stations to it. 

The Master Lake Stations were: 

o Lake Ontario: Oswego, New York 

o Lake Erie: Fairport, Ohio 

o Lake St. Clair: St. Clair Shores, Michigan 

o Lake Huron: Harbor Beach, Michigan 

o Lake Michigan: Harbor Beach, Michigan

o Lake Superior: Marquette, Michigan

Note that since this calculation, the station at Fairport has been determined to be vertically 

unstable due to nearby mining operations and will most likely be replaced by Cleveland. Each 

HC was obtained by subtracting the MWL at the Master Station (MWLMaster) from the MWL at 

the subordinate gauging station in question (MWLSub). The answer retains the arithmetic sign. 

The HC may be positive or negative and is subtracted algebraically. Figure 7 shows the HC 

values for each station. The geographic variations in HC provide information on lake level 

dynamics and lake surface mean differences. These HCs will need to be calculated during the 

next IGLD update period; for instance over the reference period 2017 through 2023 for an IGLD 

2020 update that is currently being proposed by NOAA. 
16 

Figure 7. Map of values of the Hydraulic Correctors (meters) for U.S. and Canadian water level stations. 

2.8 HC Interpolation Model 

A gap in information since implementation of IGLD 85 has been the lack of HC information 

between existing stations. The seasonal gauging program was not activated as part of IGLD 85 but 

would have provided much needed information in between the long-term stations. Because of their 

complex character and geospatial variability, HC estimates at locations other than the long-term 

operating stations have used interpolated HCs compiled using modeled data grids for potential use 

prior to future seasonal gauging programs. The NAVD 88/IGLD 85 datum transformation model 

developed by the National Geodetic Survey spatially interpolates the HC for each location desired. 

The interpolation-based compilation for HCs; using the existing stations has also been 

incorporated into the NOAA VDatum product suite (see http://vdatum.noaa.gov/). 

The fitting process for the data grids was chosen to honor the Master Station and U.S. and 

Canadian HC values determined from water level station observations over a seven-year 

summer-month period, 1982-1988. A post-fit test that re-interpolated the HCs at the Master 

Stations was shown to agree, with less than 2-mm error. Instrumental error and spatial variation 

of the HCs will prevent the complete transformation from being more accurate than a few 

centimeters. The present interpolation model’s performance at locations apart from the existing 

station networks has not been evaluated and the proposed NOS seasonal gauging program is 

designed to provide actual measured intermediate data points for an improved interpolation 

model than presently exists. One of the major outcomes of the IGLD update and NOAA’s 
17 

seasonal gauging program will be to evaluate the model performance using an increased number 

of observational points. 

2.9 Low Water Datum (LWD) – Chart Datum 

Each of the Great Lakes and Lake St. Clair have individual fixed Low Water Datum (LWD) 

reference elevation, also used as Chart Datum. The LWD/Chart Datum are chosen as 

conservative datums for navigation purposes such that during the navigation (ice-free) season the 

actual water levels in each lake are above the Chart Datum most of the time. LWD is defined by 

analysis of historical water levels during the summer months June through September at the 

Master Water Level Stations on the Great Lakes. The heights of LWD are presently computed 

relative to IGLD 85 and are shown in Figure 8 for the entire Great Lakes-St. Lawrence River 

System. All subordinate lake stations are hydraulically corrected, adjusted to reference the 

(Dynamic Height) mean water level of each lake’s corresponding Master Station. 

Figure 8. IGLD 85 Heights (m) of LWD for the Great Lakes – St. Lawrence River System. 

NOAA is transitioning into performing Ellipsoidally Referenced Hydrographic Surveys (ERS) 

for nautical charts and ellipsoidally referenced Lidar surveys for shoreline delineation. For these 

types of surveys in the Great Lakes, elevation differences between the elevation of Chart Datum 

(LWD) and the ellipsoid (NAD83) must be precisely known everywhere in the survey area. 

NAD83 is an expression of the mathematically-defined ellipsoidal reference surface, which has 

slopes and variations relative to the Earth’s surface. The relationship of LWD to the ellipsoid has 
18 

been estimated using static GPS surveys on bench marks at most NWLON stations; however, 

additional repeat surveys are needed to evaluate the GPS data for consistency, both 

geographically and over time. 
19 

3.0 GAP ANALYSIS APPROACH 

Continued operations and monitoring in the Great Lakes described in the previous sections 

require accurate water level measurements and derived products relative to an accurately 

established vertical reference datum. The necessary measurements require a network of stations 

to provide spatial and temporal coverage of the water level variations that meet a variety of needs 

including the following: real-time water levels for safe navigation; data products of daily, 

monthly and annual means for regulation and monitoring; determination of long-term decadal 

trends; and long-term data series to monitor crustal movement. The stations are also used 

operationally, providing real-time data and derived data products for storm surge warnings, 

restoration projects, and many other coastal zone management applications. The stations located 

in the connecting channels have very specific location-based requirements for monitoring flows, 

vertical control for dredging operations and hydrographic surveys, and for safe and efficient 

navigation. 

As of 2014, NOAA’s National Ocean Service operates 53 long-term water level monitoring 

stations in the Great Lakes as part of the larger NWLON operating along all U.S. coasts. Since 

water levels in the Great Lakes are largely driven by meteorological factors, a robust network of 

meteorological sensors is required to fully understand lake level dynamics. After a recent tenyear effort, installation of meteorological sensors (winds, barometric pressure, relative humidity, 

and air/water temperature) has been made at 24 of the 53 U.S. water level stations. The presence 

of the GIA signal across the region requires accurate monitoring and determination of rates of 

vertical land motion. One important system to enable this is a network of CORS. Thirteen (13) 

U.S. stations now have co-located CORS. The Canadian network includes 34 long-term stations, 

nine of which have CORS. USACE operates several long-term and short-term stations as well 

depending upon their projects. 

The GIS analysis approach used by Gill and Fisher (2008) for the NWLON gaps analysis report 

is not applicable to a similar analysis for the Great Lakes. That approach was possible due to the 

existence of error budget equations and existing overlays of co-tidal lines that produced 

estimated polygons of coverage whose sizes were driven by threshold errors for tidal datum 

determination. In this report, the approach is taken to separately assess the water level network 

needs for operational determination of lake-wide averages, for datum elevation variability and 

hydraulic correctors, for determination of rates of vertical land velocities, and for determination 

of LWD reference elevations relative to the ellipsoid. These assessments are looked at in the 

context of each lake to make recommendations and findings for the network. 

This report is a preliminary step by NOAA to assess the Great Lakes water level station network. 

Gaps in network coverage are assessed by determining if the present station location provides the 

information required to understand the lake wide variations and provide the datum and data 

products necessary for management of the Great Lakes system. It is hoped that this report can 

inform any other observing system analyses underway or in planning. 

3.1 Assessment of Lake Averages 

In a status assessment of Great Lakes-St. Lawrence River Water Resources (Great Lakes 

Commission, 2003), it was noted that daily and monthly lake-wide averages are reported to the 
20 

nearest centimeter and determined through professional judgment that the uncertainty in Great 

Lakes levels may range from 0.003 m to 0.006 m. NOAA (CO-OPS 2013) considers that 

individual six-minute measurements from the water level gauges relative to IGLD datum have 

uncertainties of +/-0.006 m and monthly mean water levels have uncertainties of +/-0.003 m. 

Neff and Nicholas (2005) estimate that the uncertainty in the normalized daily mean lake-wide 

average data is likely to be in the range of 0.002 m. These uncertainty estimates are likely 

underestimates, as they are based primarily on the uncertainty of water level gauge instruments 

and measurements at individual stations and did not take into consideration the uncertainty 

resulting from temporal and spatial variability of water levels on each lake, which is very 

important in determining uncertainty in lake-wide averages (Bruxer, 2010). The uncertainty in 

change-in-storage calculations depends on the accurate measurement of the change in lake stage 

and surface area of each lake. Thermal expansion and contraction of water also contributes to 

uncertainty in estimates of change in storage. 

Beginning-of-month (BOM) lake levels are used to determine monthly rates of storage change 

for operational purposes such as navigation, hydro-electric power, and lake regulation. BOM 

levels are computed using the daily mean water levels for the first and last days of the month for 

each network of water level stations on a given lake. Computation of lake-stage data can be 

complicated primarily by wind set-up and seiche and secondarily by GIA effects. Wind set-up 

and seiche can cause great variability in lake stage at different parts of a lake; wind set-up is 

greatest in the fall and winter, when wind tends to be strongest and more prevalent (Croley, 

1987). Due to these effects, a straight average of water levels from all of the stations on each lake 

is not used. One method used to correct for these factors is to measure lake stage at selected 

locations and average the values in a weighted fashion to determine an average lake stage 

(Croley, 1987a, 1987b). 

Quinn et al. (1974-1976) recommended using mean water levels for the two days—the first day 

of each month and the last day of the previous month—and employing a weighting function 

strategy with a Thiessen polygon procedure. To support this weighting function strategy, Quinn 

and Todd (1974) performed initial error analyses for the dependency of network size on the 

determination of BOM water levels. Through a series of similar analyses for each lake, it was 

found that placement (geographical distribution) of the gauges is a key factor in successful 

computation of BOM water levels. In a series of reports analyzing network size requirements of 

BOM lake levels, Quinn et al. (five reports: 1974 through 1976) found that the optimum number 

of gauges (both U.S. and Canadian) would be: 

Nine (9) gauges for Lake Superior 

Five (5) gauges for Lake Michigan 

Ten (10) gauges for Lake Huron 

Four (4) gauges for Lake St. Clair 

Nine (9) gauges for Lake Erie 

Nine (9) gauges for Lake Ontario 

Croley (1987) evaluated optimum network sizes using a spatial-optimization estimating 

technique for Lakes Superior and Erie and evaluated his results against the Thiessen polygon 

method developed by Quinn et al. Croley determined that, for lake level estimation purposes, to 

obtain a maximum tolerable error of 0.006 m, an eight -gauge network would be sufficient for 
21 

Lake Erie and a network of seven gauges is sufficient for Lake Superior. Croley suggests that 

the spatial-optimum estimator is superior to the Thiessen polygon method because it eliminates 

long-term wind set-up errors more effectively. 

Zervas (1997) performed an accuracy study of Great Lakes mean water levels for various 

network configurations using an empirical orthogonal function (EOF) approach. Results were 

obtained for mean lake levels using two-day averages. Findings include that only small error 

reductions are obtained for networks greater than eight (8) gauges on each lake. Small errors in 

mean lake level were classified as those with less than 0.01 m standard error and maximum 

errors less than 0.10 m. Using hourly data, Zervas also found that lake level differences 

determined by any of the spatial optimum, Thiessen, or EOF methodologies were similar (within 

0.02 m standard error and 0.15 m maximum error). Lake Erie errors are larger than the other 

lakes because the differential effects of wind stress on Lake Erie are much larger than the other 

lakes. 

Appendix 1 contains hourly water level plots and hourly lake averages for each lake for the year 

2005 to provide a sense of the lake-wide variability over a full year. These plots provide a visual 

sense of the variability of water levels for a given calendar year with the swath of simultaneous 

data plotted for each lake. November and December were particularly “noisy” due to a series of 

weather events. The hourly average plots are constructed by averaging the simultaneous hourly 

water levels for all Canadian and U.S. stations in each lake and computing the standard deviation 

of the averages for each hour. Lake Ontario is the “quietest” lake with very small standard 

deviation. Lake Erie is the “noisiest” and Lakes Michigan and Huron are very similar. The 

interesting feature for Lake Superior is the lake-wide jump in water levels in October 2005, when 

levels rose 12 cm in three days from the third to the sixth days of the month due to heavy 

precipitation. 

3.2 Assessment of Daily Means 

Lake levels are very dynamic surfaces and respond to daily weather patterns, comparisons of 

simultaneous daily mean water levels are also instructive in understanding the varying slopes of 

the lake surfaces relative to IGLD. In the assessment sections that follow, example plots of 

simultaneous daily mean values from individual stations for each lake are compared to the 

official composite daily means calculated by USACE and Environment Canada from a subset of 

stations in each lake for a one-month period (June 2013). The nature of the differences in daily 

variability among stations is examined in these plots. Figure 9 shows a monthly comparison of 

daily means for Lake Superior for June 1985 and the present (June 2013). All station values plot 

closer to the lake-wide mean in 1985 than in 2013. The elevations were adjusted to be close 

together with the application of HCs and 1985 was the center year of the seven-year IGLD 1985 

update period. Seasonal and monthly meteorologically driven biases in lake levels among 

stations and lake dynamics account for some differences; however, the larger separation biases 

between the same stations in 2013 are also related to the ongoing effects of differential rates of 

vertical land movement among the stations. The elevation differences among the stations shown 

over a one-month period in Figure 9 are consistent with longer period statistical comparisons 

(see following section). These differences are found in similar plots in the following sections for 

each individual lake. At shorter time steps, the daily variations are often negatively correlated 
22 

(oscillations out-of-phase) across the lake due to local meteorological effects as seen in both time 

periods. 

Figure 9. Comparison of daily mean water levels for June 1985 and June 2013 in Lake Superior. 

3.3 Assessment of Station Pair Comparisons 

One of the statistical analysis procedures used in this report is performed by using hourly heights 

because they are used to compute subsequent data products such as daily and monthly mean lake 

levels. Hourly heights are used rather than higher rate data such as six-minute interval data for 

ease in analysis and because the hourly and six-minute data have the same statistical 

distributions. In addition, it is the hourly data that are used to derive many subsequent products. 

For this statistical analysis the seven-year period from 2005 through 2011 was selected. A sevenyear period was selected because it is the length of time currently used for the determination of 

lake-wide datums, mean values, and Hydraulic Correctors. For many applications, only the data 

from the seasonal low water level months are used over seven-year period (i.e., 1982-1988 for 

IGLD 1985 Hydraulic Correctors and for LWD); however, statistical comparisons of hourly 

heights over twelve months for each year of a seven-year period were performed here as a 

measure of the total water level variability among various U.S. station-pairs on each lake and the 

connecting waterways. A full year of data is used also to be compatible with the Zervas (1997) 

and Mainville and Craymer (2005) analyses, which also used full years of data. 

Station pairs were generally determined by comparing neighboring stations, but also by pairing 

stations from each end of the lake to obtain a sense of maximum variability. By analyzing the 

mean differences, standard deviations, maxima and minima, and correlation coefficients from 

each comparison, a relative sense of insufficient and/or overlapping control can be obtained. For 

an initial assessment, only small mean differences would be expected because the data from the 

stations are all adjusted prior to dissemination by applying each station’s HC that was developed 

in 1985 to reference everything to IGLD. However, this assumption does not hold true due to the 

relative differences in vertical land movement between station pairs as discussed for each lake in 

following sections. Comparisons with Canadian stations and Canadian shore station pairs are 

also included for lake-wide context. It is noted that Lakes Michigan and Huron are connected via 

hydraulic strait and are considered as one basin for purposes of hydraulic correctors and lake-
23 

wide averages. Here we separate the discussion for each lake basin in order to focus the 

characterization of each lake. 

A functional observing system network provides for an appropriate amount of overlap in 

coverage such that if one station was temporarily lost, another could substitute if certain 

statistically valid adjustments could be made. For station statistical comparisons, it was generally 

found that good overlap could be defined as when in seven-year hourly height comparisons, 

station pairs have mean differences of less than 0.010 m (after accounting for vertical velocity 

differences), standard deviations of less than 0.020 m, maximum and minimum differences of 

less than 0.500 m, and linear regression coefficients of 0.99 and higher. Although it depends 

upon the individual lake, in general, network weaknesses and gaps are identified by station pairs 

having weaker statistics with mean differences of greater than 0.030 m, standard deviations of 

greater than 0.060 m, maxima and minima greater than +/-1.000 m and correlation coefficients 

less than 0.900. 

The linear regression coefficients from the recent seven-year period are then compared to the 

correlation analyses performed by Zervas (1997) to help pinpoint areas of overlap and weakness 

in network coverage for purposes of establishing datum control and mean lake levels. Zervas 

used a one-year period of hourly heights from 1990 for his analysis. The following (figures 10 

and 11) is an example of a nearby station pair with relatively high statistical correlation: 
24 

Point Iroquois and Gros Cap, Lake Superior 

7- year Mean Difference: 0.006 m 

Standard Deviation: 0.001 m 

Least Squares Linear Regression Coefficient: 0.996 

Zervas Correlation Coefficient: 0.99 

Figure 10. Hourly height differences between Point Iroquois and Gros Cap, 2005-2011. 

Figure 11. X-Y plot scatterplot comparison of hourly heights for Gros Cap and Point Iroquois. 
25 

The following (figures 12 and 13) is an example of a nearby station pair with relatively poor 

statistical correlation. Note the non-linear feature in the upper right hand corner of the graphic 

and is most likely due to the unique water level variations in Saginaw Bay (see section 4.3). 

Harbor Beach and Essexville, Lake Huron 

Mean Difference: -0.021 m 

Standard Deviation: 0.095 m 

Least Squares Linear Regression Coefficient: 0.780 

Zervas Correlation Coefficient: <0.90 

Figure 12. Hourly height differences between Harbor Beach and Essexville, 2005-2011.
26 

Figure 13. X–Y scatterplot comparison of hourly heights at Harbor Beach and Essexville, 2006-2010. 

The mean differences calculated found in tables in the next section for each station pair can 

largely be accounted for by the relative difference in vertical movement rates between station 

pairs. This assumption is based on the vertical velocity values in Figure 3 for each station; 

projected mean elevation change for each station can be estimated by multiplying the vertical 

velocity rates by 0.23 century (the portion of a century between 1985 and 2008, the central year 

of the 2005-2011 period) and then differencing the projected elevation between stations. 

For example ,(see table 1): 

Iroquois-Duluth: Duluth is falling at about 23.5 cm/century relative to Iroquois. 

(0.23)x (- 23.5) = -5.8 cm 

Therefore, the land at Duluth has fallen about 5.8 cm relative to Iroquois since 1985. As a result, 

the water levels being recorded there now are about 5.8 cm higher than at Iroquois. Therefore, 

the mean difference is projected to be Iroquois – Duluth = -0.058 m. The calculated mean 

difference from the 2005-2011 hourly data sets is -0.051 m (Table 1). 

Projected values using this procedure were obtained for each station pair, and they are listed in 

italics next to the calculated mean difference in each of the tables of statistics for each lake. In 

general, the projected and calculated values agree within expected uncertainties given the 

uncertainties in the procedure used and due to actual dynamical differences in mean lake level 

between the station pairs. 

3.4 Assessment of Hydraulic Correctors 

The distribution of the observed and modeled HCs for each lake is examined for adequate 

network distribution for datum determination and transfer. Contour maps of the HC distributions 

are presented for each lake in the following sections and assessed for their spatial variations, 
27 

anomalies, and for areas in which HCs are not well-understood. The values of the hydraulic 

correctors used for interpolation were those published by the Coordinating Committee (1995). 

Interpolation of HCs to specific project locations for which directly calculated values from a 

water level gauge are not available is an ongoing requirement for datum control. Lack of 

observation points from a distributed network throughout a given lake will negatively affect 

being able to interpolate the HC variations with any accuracy. 

3.5 Assessment of Variations of Crustal Movement 

As discussed earlier, GIA is an ongoing, significant differential vertical land motion 

phenomenon that has the northwest-southeast axis running though the lakes, about which uplift is 

occurring northeast of the axis and subsidence southwest of the axis. The impact of GIA on the 

apparent water levels is recorded at each station and over time, the differential rates of vertical 

land motion affect the hydraulic flow characteristics of the Great Lakes system and drive 

decisions for updating vertical reference datum systems. 

The distribution of the water level network stations are examined for adequacy in determining 

and monitoring the geospatial variability of the GIA. It is envisioned that as the CORS network 

co-located at the water level stations expands and accrues longer time series, the measured 

vertical velocities from CORS will have the accuracy required to better define this geospatial 

variability than using long-term water level variations. Among other reasons, the present density 

of Great Lakes stations should still be maintained in order to monitor the vertical land motion 

and the effects on Great Lakes flows and hydrology. In this study, each lake is examined for any 

redundancy in the rates of overlap between the stations on that lake. Continued monitoring will 

enable a more precise GIA model and will enable precise comparisons between using lake level 

data and CORS data to refine actual gradients in vertical land motion. However, the use of a GIA 

model is not envisioned to replace requirements of actual long-term water level measurements 

for the purpose of estimating vertical land motion even if to the present uncertainties in the 

models is improved. The water levels themselves are best at describing the land-water 

relationship around each lake, and levels feed into and can help validate other GIA models as 

well as the CORS data. The water level measurements also capture the impacts of more than just 

GIA (e.g., the water levels recorded at Fairport helped identify the subsidence issues there). Use 

of CORS data to independently measure vertical land motion requires installation of CORS at 

many more water level stations and awaits longer time periods of measurement to obtain precise 

estimates. 

Vertical velocities at each station relative to their respective lake outlets are assessed by referring 

to the values and graphics from Mainville and Craymer (2005). Bruxer and Southam (2008) also 

obtained similar estimates using water level data only from the four seasonal low-water months 

(June-September). The variations in vertical velocities among adjacent station pairs are examined 

and compared to the computed mean differences found over a recent seven-year period of hourly 

heights. 

3.6 Assessment of Ellipsoidal Height Relationships to Low Water Datum 

Using GPS surveys at each of the NWLON stations, graphs of the LWD elevation relationships 

to the ellipsoid are constructed for each lake for the U.S. gauges. There are significant and 
28 

varying slopes in the elevation differences across the lake and amongst station pairs; however 

this information is based on limited numbers of GPS surveys. These slopes and variations could 

be due to limitations of the GPS surveys or could be actual variation in the two surfaces across 

the lake shore. Additional GPS surveys are required at these stations to get a more accurate 

assessment of these apparent slopes and differences. The addition of more co-located CORS and 

water stations will also assist this effort. The seasonal gauging program planned as part of the 

IGLD update will assist in this determination by providing intermediate data in between existing 

NWLON stations. 
29 

4.0 INDIVIDUAL LAKE ASSESSMENTS 

4.1 Lake Superior 

Figure 14. U.S. and Canadian Water level Station locations for Lake Superior. 

Figure 14 (above) shows the locations of the U.S. and Canadian water level station networks for 

Lake Superior. Figure 15 is an example plot of the daily mean water levels from the subset of 

stations used on Lake Superior to compute lake-wide means for various applications. For the 

time period plotted for June 2013, three of the five stations track the lake-wide average within a 

few centimeters, however the two stations at opposite ends of the lake are several centimeters 

above and below the lake-wide average. This systematic difference is due to the impact of GIA 

on the land-to-water relationship around Lake Superior over time (See Figure 18). Duluth is 

subsiding at about 25 cm/century relative to the lake outlet (and its five-gauge lake-wide average 

level). As a result the water levels recorded there are rising at the same rate over time. The 

opposite is true at Michipicoten. The differences in Figure 15 for Point Iroquois, Marquette and 

Thunder Bay are consistent with the changes seen relative to the lake’s outlet and its five-station 

average. 

The differences in phase of the daily water level fluctuations recorded at different locations, 

(e.g., Thunder Bay water levels go up when Point Iroquois water levels go down, and vice versa) 

are indicative of the short-term seiche effects that are meteorological in origin. The slopes of the 

lake surface are constantly changing. 
30 

Figure 15. Daily mean water levels for Lake Superior for June 2013. 

Except for the pairing of the two stations at the extreme ends of the lake, the station pair 

statistical differences (Table 1 and Figure 16) between neighboring pairs are 0.03 m or less with 

linear regression coefficients near or above 0.90 for long-term hourly height comparisons. 

Correlation coefficients found by Zervas (1997) are very similar using only one year of hourly 

data from 1990. The station pair of Point Iroquois (U.S.) and Gros Cap (Canada) are located 

close to each other and show a high degree of correlation (>0.99) and very small statistical 

differences over the seven-year period. A cross-lake pair is also included here (Ontonagon-Grand 

Marais) to assess a cross-lake difference over a relatively short distance. Projected mean 

differences using the differences in vertical velocities between station pairs are listed in italics 

next to each calculated difference over the seven-year period.
31 

Table 1. Comparison statistics for Lake Superior station pairs using seven years of hourly height data from 2005-

2011. Elevations were obtained from U.S. and Canadian databases relative to IGLD 1985 ( HC applied). Projected 

mean differences are in italics. 

Lake Superior

Station Pairs Mean Difference 

(m) 

Standard 

Deviation (m) 

Max. 

Difference (m) 

Min. 

Difference (m) 

Regression 

Coefficient 

Point Iroquois-Duluth -0.051 (-0.058) 0.110 0.893 -0.707 0.612 

Point Iroquois-Marquette -0.022 (-0.028) 0.053 0.542 -0.538 0.900 

Marquette-Ontonagon -0.013 (-0.015) 0.042 0.274 -0.468 0.928 

Ontonagon-Duluth -0.016 (-0.015) 0.055 0.350 -0.358 0.890 

Duluth-Grand Marais 0.031 (0.041) 0.042 0.350 -0.244 0.937 

Ontonago-Grand Marais 0.015 (0.026) 0.034 0.283 -0.219 0.955 

Grand Marais-Thunder Bay 0.023 (0.023) 0.031 0.313 -0.269 0.961 

Thunder Bay-Rossport 0.05 (0.06) 0.04 0.49 -0.25 0.942

Rossport-Michipicoten -0.01 (-0.01) 0.04 0.33 -0.59 0.920

Michipicoten-Gros Cap -0.04 (-0.05) 0.05 0.44 -0.46 0.925

Gros Cap-Point Iroquois -0.01 (0.0) 0.01 0.19 -0.14 0.996

Figure 16. Lake Superior correlation coefficients from Zervas (1997). 
32 

The Master Station for determination of HCs in Lake Superior is Marquette, Mich. (Coordinating 

Committee, 1995). Interpolated HC contours (see Figure 17) are oriented in a general 

longitudinal direction with maximum values (≥0.10 m ) at each end of the lake. Despite the large 

amplitudes, there do not appear to be any areas with complex HC contour patterns that are seen 

in other lakes. 

Figure 17. Interpolated HCs for Lake Superior. 

The vertical land motion velocities in Lake Superior relative to the lake outlet into the St. Marys 

River are shown in Figure 18. The values range from highly negative rates (-25 cm/century) in 

the western end at Duluth to +27 cm/century in the northern end (Rossport). This is due to the 

fact that the land is rising faster as one moves to the north and east. The 0 cm/century line (which 

passes through the lake’s outlet [Figure 2]) is simply a result of this differential movement and 

the orientation of the lake itself (since the outlet is at the far eastern end of Lake Superior). There 

are very large changes in the vertical velocities over relatively small distances. CORS have been 

established at U.S. stations Marquette (in 2004), Grand Marais (in 2002) and Point Iroquois (in 

2003) and at the Canadian stations at Rossport (in 2002) and Michipicoten (in 2009); however, 

the data series are not yet long enough to accurately estimate vertical velocities for comparison 

with those from the GIA model and from the water level records. 
33 

Figure 18. Relative vertical velocities (in cm/century) for Lake Superior (Mainville and Craymer [2005]) relative to 

the lake outlet. 

Differences in the ellipsoidally based elevation surface of NAD 83 relative to LWD shown in 

Figure 19 are approximately 8.0 m from the western to eastern ends of Lake Superior. These 

preliminary values are based on static GPS surveys to individual bench marks at each station. 

This elevation difference between the surfaces could be better refined with more GPS surveys 

between Ontonagon and Marquette. The amplitudes of the unknown variation in the elevation 

differences are significant for hydrographic surveying and would need to be known prior to 

successful surveying on the ellipsoid on Lake Superior. 

Figure 19. Relationship of LWD and NAD 83 for Lake Superior. 
34 

4.2 Lake Michigan 

Figure 20. U.S. water level station locations for Lake Michigan and vicinity. 

Lakes Michigan and Huron are hydraulically connected and are treated together for most lake 

level and hydraulic corrector purposes. They are considered separately here for purposes of 

focusing in on each lake basin. Figure 20 (above) shows the locations of the U.S. water level 

stations on Lake Michigan. The statistical analyses in Table 2 show that the station pairs 

Milwaukee-Calumet and Calumet–Holland both have correlation coefficients below 0.90 and 

have relatively high standard deviations in their mean differences. In Figure 21, the convention 

of Zervas (1997) is to show lines between stations only if the correlation is greater than or equal 

to 0.90. The lack of lines depicted between those same two station pairs is because the 

correlations are less than 0.90, which is consistent with the results from Table 2. Lines are not 

shown between Green Bay and any other station, or between Ludington and Mackinaw City. 

This is consistent with the large differences and standard devations and relatively small 

regression coefficients in Table 2. The calculated mean differences are presented in Table 2 

along with the projected mean differences based on the relative differences in vertical velocities 

of land motion of the station pairs. Vertical velocities are not available for Menominee, as it is a 

relatively new station. Projected mean differences using the differences in vertical velocities 

between station pairs are listed in italics next to each calculated difference over the seven-year 

period. The mean difference is relatively high and the correlation coefficient is relatively low for 

the Ludington-Mackinaw City station pair.
35 

Table 2. Comparison statistics for Lake Michigan station pairs using seven years of hourly height data from 2005-

2011. Elevations were obtained from U.S. and Canadian databases relative to IGLD 1985 ( HCs applied). Projected 

mean differences are in italics. 

Lake Michigan

Station Pairs Mean Difference 

(m) 

Standard 

Deviation (m) 

Maximum 

Difference (m) 

Minimum 

Difference (m) 

Regression 

Coefficient 

Mackinaw City-Calumet -0.073 (-0.047) 0.120 0.815 -1.162 0.613 

Mackinaw City-Port Inland -0.017 (-0.001) 0.051 0.314 -0.458 0.923 

Port Inland-Menominee -0.015 (n/a) 0.067 0.711 -0.505 0.888 

Menominee-Green Bay -0.019 (n/a) 0.081 0.744 -1.096 0.859 

Port Inland-Sturgeon Bay -0.027 (-0.030) 0.054 0.595 -0.377 0.917 

Menominee-Sturgeon Bay -0.012 (n/a) 0.044 0.354 -0.333 0.950 

Sturgeon Bay-Kewaunee 0.000 (-0.010) 0.034 0.340 -0.362 0.962 

Kewaunee-Milwaukee -0.020 (-0.014) 0.050 0.533 -0.510 0.919 

Milwaukee-Calumet -0.008 (+0.009) 0.064 0.599 -0.616 0.888 

Calumet-Holland 0.007 (0.006) 0.068 0.564 -0.609 0.873 

Holland-Ludington 0.003 (-0.010) 0.044 0.571 -0.359 0.938 

Ludington-Mackinaw City 0.062 (0.051) 0.063 0.326 -0.335 0.869 
36 

Figure 21. Lake Michigan correlation coefficients from Zervas (1997). 

Network coverage in Green Bay is complicated by the active high frequency oscillation in that 

bay. Zervas (1997) found from EOF analyses that Green Bay variations dominated the second 

modes, indicating that it has a large water level signal unique to that location. Figure 22 further 

illustrates the nature of the anomlaous water levels in Green Bay relative to the rest of Lake 

Michigan using an example simultaneous plot of data from May and June 2013. With Sturgeon 

Bay representing the outside lake, the plots of Menominee and Green Bay show a regular high 

frequency oscillation (10 to12 hour period) that increases in amplitude going up into Green Bay. 

This is symptomatic of a resonance or amplification of the normal lake sieche by the bay 

configuration. The bathymetric contour map (Figure 23) shows how Green Bay is isolated from 

the rest of the lake by shallower water at the entrance. Quinn (1975) also considered this 
37 

anomalous area for purposes of computing lake-wide means by assigning a weight of only 0.037 

for a limited geographic Thiessen polygon. The relatively recent establishment of the station at 

Menominee (not inlcuded in the Zervas (1997) work) has bolstered the network in that region. 

Figure 22. Simultaneous plot of data from Green Bay, Menominee and Sturgeon Bay for May and June 2013. 
38 

Figure 23. Bathymetric contours for Green Bay, Lake Michigan (from: 

http://www.ngdc.noaa.gov/mgg/greatlakes/lakemich_cdrom/html/area1.htm). 

The orientation of the interpolated HCs is in a longitudinal direction, increasing in value from 

east to west (see Figure 24). The Master Station from which the HCs are determined is located at 

Harbor Beach in Lake Huron. The actual location of these HC contours is not well known along 

the northeastern shore of Lake Michigan due to lack of measurement points between Ludington 

and Mackinaw City, Lake Huron. 
39 

Figure 24. Interpolated Hydraulic Correctors (in meters) for Lake Michigan.

The main feature in the distribution of vertical land velocities in Lake Michigan relative to the 

Lake Huron outlet is their change from negative velocities (0–14 cm/century) in the southern 

portion to positive velocities (0-10 cm/century) in the northern portion (see Figure 25). This 

pattern is caused by the varying differences in velocities of vertical land movement. The GIA 

tilting axis for the Great Lakes region (see Figure 2) also transects the lake. As mentioned, these 

velocities are relative to the lake outlet from Lake Huron because the two lakes are considered 

hydraulically connected through the Straits of Mackinac. 

Figure 25. Relative vertical velocities (in cm/century) for Lake Michigan and Lake Huron (Mainville and Craymer 

[2005]) relative to the lake outlet. 
40 

CORS have been established at U.S. water level stations at Calumet (in 2002), Ludington (in 

2004), and Mackinaw City (in 2009) and do not yet have a sufficiently long data series to 

determine accurate vertical velocities. Near-term planning includes potential installation of new 

CORS at Holland and Menominee.

Differences in the ellipsoidally based elevation surface of NAD 83 relative to LWD shown in 

Figure 26 vary by approximately 4.0 m from the southern to northern ends of Lake Michigan. 

Additional information about the differences will be added when GPS survey information 

becomes available at Holland. The value for Calumet Harbor is an apparent anomaly; however, it 

may be a true estimate as that is the southernmost station on the lake. The amplitudes of the 

unknown variation in the elevation differences are significant for hydrographic surveying and 

must be known prior to successful surveying on the ellipsoid on Lake Michigan. The values 

presented are based on static GPS surveys to one bench mark at each station. 

Figure 26. Relationship of LWD to NAD83 for Lake Michigan.
41 

4.3 Lake Huron 

Figure 27. U.S. and Canadian water level station locations for Lake Huron and vicinity. 

Figure 27 (above) shows the locations of the U.S. and Canadian water level station networks for 

Lake Huron. From the plots of daily mean water levels for June 2013 from Lake 

Huron/Michigan (Figure 28), it can be seen that Harbor Beach water levels track the lake-wide 

mean closely (Harbor Beach is located geographically close to the lake outlet and has very 

similar vertical land movement velocities to the outlet). The daily means for northern Lake 

Huron track below the lake-wide mean by approximately 5 cm during this period. The daily 

means for the stations in Lake Michigan track above the lake-wide mean by approximately 5 cm. 

These differences are due to the impact of differential vertical land movement in the lake basins. 

In general, differences and standard deviations are higher and regression coefficients lower for 

the Canadian station pairs, reflective of the two sub-basins in northern Lake Huron. Projected 

mean differences using the differences in vertical velocities between station pairs are listed in 

italics next to each calculated difference over the seven-year period. 
42 

Figure 28. Lake Michigan/Lake Huron Daily Mean Water Levels June 2013. 

Table 3. Comparison statistics for Lake Huron station pairs using seven years of hourly height data from 2005-2011. 

Elevations were obtained from U.S. and Canadian databases relative to IGLD 1985 (HCs applied). Projected mean 

differences are in italics. 

Lake Huron

Station Pairs Mean Difference 

(m) 

Standard 

Deviation (m) 

Maximum 

Difference (m) 

Minimum 

Difference (m) 

Regression 

Coefficient 

Lakeport-Mackinaw 0.041 (0.023) 0.100 0.899 -0.707 0.702

Lakeport-Harbor Beach 0.006 (0.000) 0.041 0.539 -0.477 0.949 

Harbor Beach-Essexville -0.021 (-0.003) 0.095 0.881 -1.182 0.780

Essexville-Alpena 0.080 (n/a) 0.120 1.455 -0.955 0.677 

Alpena-Mackinaw City -0.022 (n/a) 0.050 0.320 -0.573 0.922

Mackinaw City-De Tour 

Village 

0.009 (0.017) 0.032 0.298 -0.158 0.965 

De Tour Village-Thessalon 0.01 (0.01) 0.02 0.18 -0.20 0.989

Thessalon- Little Current 0.00 (0.01) 0.05 0.38 -0.55 0.922

Little Current-Parry Sound 0.00 (-0.01) 0.06 0.47 -0.64 0.898

Parry Sound-Collingwood -0.04 (-0.02) 0.04 0.37 -0.50 0.959

Collingwood-Tobermory 0.02 ( 0.0) 0.04 0.43 -0.43 0.938

Tobermory-Goderich -0.06 (-0.04) 0.05 0.35 -0.54 0.905 

Goderich-Lakeport 0.02 (0.00) 0.05 0.62 -0.50 0.918 
43 

Figure 29. Lake Huron correlation coefficients from Zervas (1997). The new U.S. station at Alpena has replaced 

Harrisville subsequent to his study. 

Both Table 3 statistics and figures 29 and 30 show the anomalous nature of Saginaw Bay water 

level variations as represented by Essexville. Essexville comparisons show low correlations with 

both Alpena to the north and Harbor Beach to the east. Zervas (1997) found through an EOF 

analysis that Essexville data dominates the second modes for Lake Huron, indicating that there is 

a large signal that is unique to that location. Figure 30 provides insight as to the anomalous 

nature of the water level variations in Saginaw Bay relative to the rest of Lake Huron. The time 

series plots over two months illustrate the oscillatory nature of the high water events. The 

primary modes of oscillation have much longer periods than the outside lake and appear to be 

due to localized wind set-up and bay configuration. Quinn (1975) found this as well, assigning a 

weighting factor of 0.035 for a very limited Thiessen polygon for Essexville. The bathymetry of 

Saginaw Bay (Figure 31) with depths much shallower than the rest of the lake also enhances the 

effects of wind on the water levels. 
44 

Figure 30. Water level variations for a sample period of May and June 2013 for Essexville and Harbor Beach. 
45 

Figure 31. Bathymetric contour plots for Saginaw Bay, Lake Huron (from 

http://www.michigan.gov/documents/deq/deq-whm-hwp-dow-9-2007-Draft-CCR-SRB-Fig-2-7_210883_7.pdf). 

As shown in Figure 32, there are only relatively small variations (< 0.01 m) in the HC on most of 

the lake, with the largest values up near the Straits of Mackinac. HC values are calculated 

relative to the Master Station at Harbor Beach. 
46 

Figure 32. Interpolated Hydraulic Correctors for Lake Huron.

The land in the northern portions of the lake is rising faster relative to the lake outlet on the 

southernmost end (Figure 33). The rates are especially high in the northern upper bays on the 

Canadian side, consistent with the contours for the absolute rates shown in Figure 2. CORS have 

been established at U.S. stations at Alpena (in 2006) and Harbor Beach (in 2002). In Canada, 

CORS are operating at Parry Sound (since 2002) and Little Current (since 2011).

Figure 33. Vertical velocities (in cm/century) for Lake Michigan and Lake Huron (Mainville and Craymer [2005]) 

relative to the lake outlet. 
47 

There is an approximate 2.5 m-change in the elevation difference between NAD 83 and LWD 

from south to northern portions of Lake Huron (Figure 34). Ellipsoidal elevations at each station 

are derived from static GPS surveys at individual bench marks at each station. The apparent 

anomalous value at Essexville is expected due to the anomalous nature of the water level 

variations; however the seemingly anomalous value at Mackinaw City awaits further 

confirmation from future GPS surveys. Interpolation of these values across Lake Huron may be 

sufficient for hydrographic survey uncertainty targets; however, values from all stations would 

first be required. 

Figure 34. Relationship of LWD to NAD83 for Lake Huron.
48 

4.4 Lake St. Clair 

Figure 35. U.S. and Canadian Water level station locations for Lake St. Clair and vicinity. 

Figure 35 (above) shows the locations of the U.S. and Canadian water level station networks for 

Lake St. Clair. Variations in daily mean water levels between the two stations on Lake St. Clair 

are very small (Figure 36 for June 2013), with the larger differences occuring during the time 

periods of higher frequency oscillations. 
49 

Figure 36. Lake St. Clair daily mean water levels for June 2013. 

There are only two stations on Lake St. Clair: the U.S. station at St. Clair Shores and the 

Canadian station at Belle River. Although the mean difference of simultaneous data between St. 

Clair Shores and Belle River is 0.000 m, there is a large standard deviation in the hourly 

differences and a 0.96 regression coefficient. There are large differences in mean water levels 

between St. Clair Shores and Algonac and St. Clair Shores and Windmill Point; due to the fact 

that Algonac is located upstream of the lake on the lower St. Clair River and Windmill Point is 

downstream of the lake in the upper Detroit River. Comparisons of mean differences with those 

projected by vertical land movement velocities are not available for these stations. 

Table 4. Comparison statistics for Lake St. Clair station pairs using seven years of hourly height data from 2005-

2011. Elevations were obtained from U.S. and Canadian databases relative to IGLD 1985 ( HCs required for Belle 

River only). 

Lake St. Clair

Station Pairs Mean 

Difference (m) 

Standard 

Deviation(m) 

Maximum 

Difference (m) 

Minimum 

Difference (m) 

Regression 

Coefficient 

Algonac-St. Clair Shores 0.177 0.088 0.999 -0.138 0.788 

St. Clair Shores-Windmill 

Point 

0.064 0.023 0.310 -0.124 0.986

St. Clair Shores-Belle River 0.000 0.039 0.234 -0.452 0.957 
50 

4.5 Lake Erie 

Figure 37. U.S. and Canadian water level station locations for Lake Erie and vicinity. 

Figure 37 (above) shows the locations of the U.S. and Canadian water level station networks for 

Lake Erie. The effects of seiche action on Lake Erie are illustrated in Figure 38. The daily mean 

water levels at opposite ends of the lake (Toledo and Port Colborne) are negatively correlated 

and result in a tilting of the lake surface back and forth throughout the month in response to wind 

forcing. This tilting also has longer period seasonal variations. The daily means for stations in 

the middle of the lake are near the lake average because they are located near the physical nodal 

line of the east-west basin oscillation. There is also a lesser north-south oscillation during some 

events. The mean differences are also due to the effect of differences in relative vertical land 

movement between station pairs, as was found for the other lakes. 
51 

Figure 38. Lake Erie daily mean water Levels June 2013. 

Replacement of Fairport gauging station as the Master Station for Lake Erie may be required due 

to the vertical land subsidence at Fairport resulting from mining operations. Cleveland has been 

suggested as the Master Station for the IGLD update as it was the Master Station reference for 

IGLD 1955. However, the Cordination Committee is actively discussing options. The 

correlations for Lake Erie station pairs shown in Table 5 and Figure 39 are relatively weaker than 

for other lakes due mainly to the high frequency of seiche events in the lake. The correlation 

coefficients are especially low and the mean differences are relatively high for the ClevelandErie and Fairport-Erie pairs. Differences in mean lake levels are fairly low and correlation 

coefficients are fairly high between the U.S. stations at Sturgeon Point and Buffalo and the 

closest Canadian station at Port Colborne. The analyses show an interesting feature of the 

western, central, and eastern station groups, which have relative high correlations amongst their 

geographic groupings but relatively low correlations between the three groups. This is a 

manifestation of the signficant east-west seiche action found in the lake. Projected mean 

differences using the differences in vertical velocities between station pairs are listed in italics 

next to each calculated difference over the seven-year period. Extra pairing analyses were 

performed for Port Colborne and Sturgeon Point to better understand the differences and 

similarities at the far eastern end of the lake. 
52 

Table 5. Comparison statistics for Lake Erie station pairs using seven years of hourly height data from 2005-2011. 

Elevations were obtained from U.S. and Canadian databases relative to IGLD 1985 (HCs applied). Projected mean 

differences are in italics.

Lake Erie

Station Pairs Mean Difference(m) Standard 

Deviation(m) 

Maximum 

Difference (m) 

Minimum 

Difference (m) 

Regression 

Coefficient 

Buffalo-Fermi 0.023 (0.022) 0.307 4.422 -1.756 0.040 

Buffalo-Sturgeon Point 0.008 (0.005) 0.033 0.597 -0.375 0.982 

Sturgeon Point-Erie -0.010 (-0.033) 0.070 1.334 -0.475 0.902 

Erie-Fairport -0.031 (n/a) 0.101 1.724 -0.904 0.743 

Fairport-Cleveland 0.044 (n/a) 0.041 0.687 -0.274 0.957 

Cleveland-Marblehead 0.009 (0.000) 0.084 0.942 -0.451 0.875 

Marblehead-Toledo -0.013 (0.000) 0.097 1.538 -0.738 0.896 

Toledo-Fermi 0.014 (-0.002) 0.062 0.536 -0.808 0.957 

Fermi-Bar Point -0.013 (n/a) 0.033 1.026 -0.858 0.984 

Bar Point-Kingsville 0.01 (n/a) 0.05 0.41 -0.90 0.962

Kingsville-Erieau 0.00 (0.00) 0.08 0.47 -1.19 0.886

Erieau-Port Stanley -0.02 (-0.01) 0.06 0.32 -0.69 0.916

Port Stanley-Port Dover 0.01 (+0.01) 0.09 0.70 -1.77 0.789 

Port Dover-Port Colborne -0.01 (-0.01) 0.06 0.32 -0.97 0.932

Port Colborne-Buffalo -0.001 (-0.013) 0.04 0.47 -0.67 0.968

Sturgeon Point-Port 

Colborne 

-0.007 (-0.017) 0.023 0.340 -0.271 0.989 

Figure 39. Lake Erie correlation coefficients from Zervas (1997). 
53 

The interpolated field for HCs at Lake Erie (Figure 40) show a longitudinally varying pattern 

away from the Master Station at Fairport in the center of the lake, however, the western end of 

the lake shows a more complicated latitudinal pattern that underlies uncertainty in the ability to 

interpolate. 

Figure 40. Interpolated Hydraulic Correctors for Lake Erie. 

Lake Erie has the smallest changes in vertical land velocities relative to the lake outlet (Figure 

41) of all of the Great Lakes because the vertical velocity gradient here (i.e., the differential

movement rates) are less than in the more northerly located lakes as they were covered by a 

greater weight and longer period of glaciation than Lake Erie. 

Here, the land at the western end of the lake is rising slower than the outlet at the eastern end of 

the lake, and therefore much of the lake appears to be moving downward, relative to the outlet. 

CORS have been established at U.S. stations at Buffalo (in 2002), Cleveland (in 2005), and 

Marblehead (in 2004). In Canada, a CORS is co-located at Kingsville (since 2011). A CORS is 

planned for installation at Fairport by the U.S. 
54 

Figure 41. Vertical velocities (in cm/century) for Lake Erie (Mainville and Craymer [2005]) relative to the lake outlet. 

The variation in the elevation difference between NAD 83 and LWD in Lake Erie varies only 

approximately 1.3 m from the western to eastern ends as shown in Figure 42. NAD 83 elevations 

are obtained by a GPS survey to one of the bench marks at each station. The anomalous drop in 

the value at Fairport is expected due to local subsidence; however, the source for the apparent 

anomalous value at Cleveland is unknown. Interpolation of the known values should be adequate 

for survey-on-the-ellipsoid purpose in Lake Erie; however, values for the Canadian stations 

should be first obtained to fully understand north-south variability. 

Figure 42. Relationship of LWD to NAD83 for Lake Erie. 
55 

4.6 Lake Ontario 

Figure 43. U.S. and Canadian water level Station locations for Lake Ontario. 

Figure 43 (above) shows the locations of the U.S. and Canadian water level station networks for 

Lake Ontario. Lake Ontario daily mean water levels (Figure 44) show individual station 

differences of a few centimeters from the lake-wide mean over a given month (June 2013). Some 

of this difference is due to the difference in the velocities of vertical land movment across the 

lake. The figure illustrates that Lake Ontario, which is quieter than Lake Erie in terms of natural 

variability due to seiche forcing and does not exhibit large variability in seasonal extremes from 

the long-term mean, can still have cross-lake centimeter-level slopes in water levels on any given 

day. 
56 

Figure 44. Lake Ontario Daily Mean Water Levels June 2013. 

Both Table 6 and Figure 45 show most station pairs (except for comparison of stations at the 

extreme ends of the lake) have relatively small statistical differences and high correlation 

coefficients. Figure 45 has lines present between each station pair with very high correlation 

coefficients. This suggests that a high degree of overlap is present in the existing network for 

monitoring of lake levels and determining lake level means. The station pair of Port Weller and 

Olcott in particular shows very small mean differences and very high correlation coefficients. 

The lowest regression coefficient for Lake Ontario is found for the Canadian Cobourg-Kingston 

station pair, and the largest mean difference is found for the Toronto-Cobourg station pair. 

Similar for the other lakes (and shown in italics in Table 6), the mean differences are highly 

correlated with the projected elevation differences due to the differences in velocities of vertical 

land movement at the stations. Projected mean differences using the differences in vertical 

velocities between station pairs are listed in italics next to each calculated difference over the 

seven-year period.
57 

Table 6. Comparison statistics for Lake Ontario station pairs using seven years of hourly height data from 2005-

2011. Elevations were obtained from U.S. and Canadian databases relative to IGLD 1985 ( HCs applied). Projected 

mean differences are in italics. 

Lake Ontario

Station Pairs Mean Difference 

(m) 

Standard 

Deviation (m) 

Maximum 

Difference (m) 

Minimum 

Difference (m) 

Regression 

Coefficient 

Cape Vincent-Burlington -0.048 (-0.046) 0.069 0.917 -0.768 0.922 

Cape Vincent-Oswego -0.019 (-0.010) 0.033 0.365 -0.853 0.979 

Oswego-Rochester 0.001 (-0.013) 0.032 0.790 -0.310 0.981 

Rochester- Olcott -0.008 (-0.002) 0.028 0.377 -0.229 0.987 

Olcott–Port Weller -0.007 (-0.008) 0.017 0.137 -0.185 0.995 

Port Weller-Burlington -0.02 (-0.01) 0.03 0.22 -0.51 0.988

Burlington-Toronto 0.01 (0.02) 0.03 0.48 -0.27 0.982

Toronto-Cobourg 0.03 (0.01) 0.03 0.34 -0.35 0.980

Cobourg-Kingston 0.01 (0.02) 0.05 0.34 -0.59 0.964

Kingston-Cape Vincent 0.003 (0.006) 0.014 0.209 -0.232 0.996 

Figure 45. Lake Ontario correlation coefficients from Zervas (1997). 

Differences in HCs across Lake Ontario (Figure 46) show a strong longitudinal pattern away 

from the Master Station at Oswego with very little change in the eastern two-thirds of the lake. 

The western end is more complicated and shows several centimeter differences illustrated by the 

closeness of the 0.025 m interval contour lines. 
58 

Figure 46. Interpolated Hydraulic Correctors for Lake Ontario. 

Figure 47 shows a steady increase in the rate of relative vertical land velocities from the lake 

outlet westward up to -20 cm/century (0.02 mm/yr). This is consistent with Figure 2, which 

shows large differences in absolute rates of vertical land motion in an east-west transect across 

the lake in response to GIA. All of Lake Ontario appears to be moving up relative to the center of 

the earth, the western end is just moving up at a slower rate than the eastern end, and hence 

appears to be falling relative to the outlet. CORS have been established at the U.S. station at 

Oswego (in 2005) and in Canada at Port Weller (in 2002), Cobourg (in 2011), and Kingston (in 

2002). These installations may soon provide accurate estimates of vertical land velocities after a 

few more years of data are accrued. 

Figure 47. Vertical velocities (in cm/century) for Lake Ontario (Mainville and Craymer [2005]) relative to the lake outlet. 
59 

The variation in the elevation difference between NAD83 and LWD in Lake Ontario varies 

approximately 2.0 m from the western to eastern ends as shown in Figure 48. Individual station 

values are obtained using static GPS surveys on a bench mark for each station. Interpolation of 

the known values should be adequate for survey-on-the-ellipsoid purposes in Lake Ontario; 

however, values for the Canadian stations should be first obtained to fully understand northsouth variability. 

Figure 48. Relationship of LWD to NAD83 for Lake Ontario 

4.7 Connecting Channels 

Three connecting channels (St. Marys River, St. Clair River, and the Detroit River) as well as the 

navigation channel though Lake St. Clair require continuous maintenance dredging to ensure 

safe and navigable channels. The existing network of NWLON stations in these waterways, 

supplemented by Canadian and USACE water level stations, provide the vertical control 

necessary for precise and effective dredging operations. 

Great Lakes Observing System (GLOS, 2013) assessments state that the water level network on 

the St. Marys, St. Clair, Detroit, Niagara, and St. Lawrence rivers is sufficiently dense; however, 

more information on horizontal current flows in these major river courses is required. There is 

no effort here to assess the water level network for spatial gaps, as that information is not 

required for understanding lake-wide averages and hydraulic correctors. Placement of the 

stations over time has been to fulfill the specific location-based need for maintenance dredging 

activities, navigation, and regulatory and ice management operations. 

The following information provides statistical differences over the same period as was done for 

each lake in the previous section and gives a sense of the similarities and differences of the mean 

values and their correlations. Long-term data is not available online for the USACE stations; 

however, key Canadian network stations are included in the analyses for context. 
60 

St. Marys River 

Figure 49. Location of NWLON, Canadian, and USACE water level stations on St. Marys River. Note: The station 

at Lookout #4 has been replaced by a nearby station at West Neebish Island. Little Rapids is now a NOAA-operated 

station (http://www.great-lakes.net/envt/water/levels/levels_current.html). 

USACE water level stations (Figure 49 and Table 7) complement the NOAA and Canadian 

networks so that the key have coverage of the narrow and depth limited navigation channels. 

There are two Canadian and two U.S. stations upstream and downstream of the locks that 

provide direct control for lock operations. The station at Little Rapids is a recent addition to the 

NWLON and was established in 2009 and a new station at West Neebish has replaced the station 

at Lookout #4. 

Table 7. Comparison statistics for St. Marys River station pairs using seven years of hourly height data from 2005-

2011. Elevations were obtained from databases relative to IGLD 1985. 

St. Marys River

Station Pairs Mean 

Difference 

(m) 

Standard 

Deviation (m) 

Maximum 

Difference 

(m) 

Minimum 

Difference(m) 

Regression 

Coefficient 

(m) 

Pt. Iroquois-S.W Pier 0.146 0.058 0.947 -0.522 0.896 

U.S. Slip-Little Rapids 

(2009-2011 only) 

0.045 0.019 0.204 -0.098 0.992 

Little Rapids-W. Neebish 

(2009-2011 only) 

0.135 0.073 0.542 -0.101 0.852 

W. Neebish-Rock Cut 0.059 0.046 0.404 -0.195 0.938 

Rock Cut-De Tour Vil. 0.032 0.044 0.469 -0.280 0.935 
61 

St. Clair River 

Figure 50. Locations of U.S. NWLON (blue), Canadian (red), and USACE (gray) water level stations on the St. 

Clair River (http://www.great-lakes.net/envt/water/levels/levels_current.html). 

USACE operates several stations to provide control for their operations on the lower St. Clair 

River to complement the U.S. and Canadian stations (Figure 50). USACE data were not readily 

available for this study to properly assess the impacts of large differences in the mean values 

between the various station pairs as shown in Table 8. There are frequent anomalously large 

differences for some station pairs during the winter months when large amounts of ice in the 

river would inhibit normal flows. 

Table 8. Comparison statistics for St. Clair River station pairs using seven years of hourly height data from 2005-

2011. Elevations were obtained from database relative to IGLD 1985. 

St. Clair River

Station Pairs Mean 

Difference 

(m) 

Standard 

Deviation 

(m) 

Maximum 

Difference 

(m) 

Minimum 

Difference 

(m) 

Regression 

Coefficient 

Ft. Gratiot-Dunn Paper 0.105 0.024 0.373 -0.129 0.986 

Dunn Paper-Point 

Edward 

0.048 0.023 0.306 -0.188 0.982 

Point Edward-Mouth of 

Black River 

0.021 0.018 0.248 -0.194 0.987 

Mouth of Black R.-Dry 

Dock 

0.098 0.022 0.275 -0.136 0.983 

Dry Dock-St. Clair Police 0.298 0.068 1.041 -0.028 0.829 

St. Clair Police-Port 

Lambton 

0.306 0.054 0.858 -0.064 0.883 

Port Lambton-Algonac 0.056 0.039 0.509 -0.042 0.932 
62 

Detroit River 

Figure 51. Locations of U.S. (blue), Canadian (red), and USACE (gray) water level stations on the Detroit River 

(http://www.great-lakes.net/envt/water/levels/levels_current.html). 

USACE operates only one additional station on the Detroit River (see Figure 51). There are 

expected significant elevation differences for all station pairs because the rivers are sloped 

surfaces. 

Table 9. Comparison statistics for Detroit River station pairs using seven years of hourly height data from 2005-

2011. Elevations were obtained from database relative to IGLD 1985.

Detroit River

Station Pairs Mean 

Difference 

(m) 

Standard 

Deviation 

(m) 

Maximum 

Difference 

(m) 

Minimum 

Difference 

(m) 

Regression 

Coefficient 

Windmill Pt.-Ft. Wayne 0.188 0.039 0.449 -0.139 0.960 

Ft. Wayne-Wyandotte 0.104 0.028 0.368 -0.095 0.982 

Wyandotte-Amherstburg 0.08 0.02 0.24 -0.02 0.993 

Amherstburg-Gibraltar 0.243 0.069 0.853 -0.086 0.935 
63 

Niagara River 

Figure 52. Location of U.S. Stations on the Niagara River (http://tidesandcurrents.noaa.gov/map/).

The stations shown in Figure 52 (and Table 8) are located at very specific locations to aid in the 

monitoring of the water levels used for international water management purposes. These gauge 

locations are part of a program to verify the gauge ratings used to determine flows in support of 

the Boundary Water Treaty of 1909 and the 1950 Diversion of the Niagara River Treaty between 

Canada and the United States of America concerning uses of the waters of the Niagara River. 

Table 10. Comparison statistics for the Niagara River station pairs using seven years of hourly height data from 

2005-2011. Elevations were obtained from database relative to IGLD 1985.

Niagara River

Station Pairs Mean

Difference 

(m) 

Standard 

Deviation 

(m) 

Maximum 

Difference 

(m) 

Minimum 

Difference 

(m) 

Regression 

Coefficient 

Buffalo-Niagara 

Intake 

2.888 0.207 5.047 1.821 0.208 

Niagara IntakeAmerican Falls 0.842 0.084 1.691 0.586 0.520 

Ashland AvenueOlcott

22.937 1.312 29.437 20.728 0.154 
64 

St. Lawrence River 

Figure 53. Location of NWLON and Canadian Stations in the international section of the St. Lawrence River. Note: 

Stations at Kingston and Cape Vincent are on Lake Ontario and are included in the analyses for that Lake. Power 

entity and Seaway Authority Stations are not included (http://www.greatlakes.net/envt/water/levels/levels_current.html) 

The station pairs in this portion of the river (Table 11) all have large correlation coefficients and 

small standard deviations. As expected on sloped river surfaces, there are significant drops in 

elevation going down the river from Lake Ontario as exhibited by the mean differences found in 

Table 11. 

Table 11. Comparison statistics for the St. Lawrence River station pairs using seven years of hourly height data 

from 2005-2011. Elevations were obtained from database relative to IGLD 1985

St. Lawrence River

Station Pairs Mean

Difference 

(m) 

Standard 

Deviation 

(m) 

Maximum 

Difference 

(m) 

Minimum 

Difference 

(m) 

Regression 

Coefficient 

Cape VincentAlexandria Bay 0.121 0.031 0.560 -0.211 0.984 

Alexandria BayBrockville 0.126 0.033 0.456 -0.127 0.980 

BrockvilleOgdensburg 0.056 0.015 0.150 -0.081 0.996 
65 

5.0 SUMMARY OF FINDINGS AND RECOMMENDATIONS 

This assessment of the Great Lakes NWLON evaluates the variability of key physical parameters 

through the Great Lakes system and assesses how well these variations are understood and 

monitored in order to meet the requirements for water level vertical control. The key parameters 

include: statistical summaries of hourly height comparisons among station pairs, comparisons of 

daily mean water levels, spatial variations in hydraulic correctors, spatial variations in long-term 

rates of vertical land motion, and cross-lake variations in the elevation differences of Low Water 

Datum to the Ellipsoid. Gaps can be identified by a lack of coverage for one or a combination of 

these parameters. In addition to the analysis of updated data sets, this analysis draws upon 

previous analyses of the variability of the key parameters in lake-wide, two-day average lake 

levels and one-year averages of hourly heights. This report intends to show the context of the 

contribution of U.S. stations to the bilateral network requirement, so the analysis thus needs to 

consider the geographical and hydrological context for each lake. Canadian stations were 

included in the statistical analyses for this purpose. 

Lake Superior 

The existing NWLON network in Lake Superior appears to be adequate in number and spatial 

distribution to meet requirements, however there is very little network overlap. The Gros CapPoint Iroquois station pair could be considered to exhibit a very high degree of overlap in 

coverage; however, there is a slight difference with their relative vertical velocities that needs to 

be monitored. Agency subject matter experts have recommended pursuing the installation of a 

new station near the northern tip of the Keweenaw Peninsula due to stakeholder requests and the 

strategic location near the geographic center of the lake. Putting a priority on installation of a 

seasonal gauge at that location would assist in understanding lake level differences with nearby 

stations and its value to understanding lake level dynamics. 

Lake Michigan/Lake Huron 

Lake Michigan has two areas of significant network weakness where gaps in NWLON coverage 

exist. The gaps in coverage are in the lower southeast corner between Calumet and Holland and 

between Ludington and Mackinaw City on the northeast shore. This assessment is based on 

using information from Table 2 and Figure 20 showing large statistical differences and low 

regression and correlation coefficients between the station pairs. Differences in vertical velocities 

are also quite large in these areas, especially along the shore between Ludington and Mackinaw 

City. Initial analyses and stakeholder contacts lead to a recommendation for new NWLON 

stations in the vincinity of Burns Harbor (in the southeast) and Charlevoix (in the northeast). 

Green Bay has significantly different water level variations than the rest of Lake Michigan due to 

a resonant seiche in that body of water. The Green Bay water level station provides necessary 

control for inside Green Bay; however, Green Bay variations are so anomalous that those data 

have limited value in computing lake-wide means for most applications. 

The existing station Kewaunee has been recommended for relocation, moving it further south to 

Manitowoc, WI for a variety of logistical and stakeholder needs. This relocation would also 

serve to better distribute the station pair differences along the western shore of Lake Michigan. 

Lake Huron appears to have adequate coverage in number and distribution to meet NWLON 

network requirements, however it is noted that Saginaw Bay (covered by Essexville) has 
66 

significantly different water level variation than the main body of the lake. Essexville provides 

necessary local control for Saginaw Bay, but the Essexville data are so anomalous that they have 

very limited value in computing lake-wide means for most applications. The northern shore 

station pairs show some relatively smaller correlations and higher standard deviations between 

station pairs. However, the differences do not appear large enough to warrant recommending 

additional stations. 

Lake St. Clair 

There is an NWLON gap in water level and datum information in the Anchor Bay region of 

northwestern Lake St.Clair as illustrated in Table 4 by the large difference in elevation, large 

standard deviation and low correlation statistics between St. Clair Shores and Algonac. 

Developing water level zoning for hydrographic survey reducers will remain challenging in this 

area until additional data are obtained. This gap is consistent with the findings of Kelley (1976) 

in which additional gauges were recommended for Lake St. Clair in the northern and eastern 

areas of the lake to increase the accuracy of BOM lake levels. The location for a new long-term 

station could be determined by placing a seasonal gauge in the area and examining the data for 

differences with the existing station at St. Clair to the south and upstream at Algonac. 

Lake Erie 

Resolving large seiche oscillations for purposes of lake–wide means and monitoring requires a 

proper spacing of stations around Lake Erie. The existing NWLON network has an adequate 

number of stations to meet these requirements; however, the distribution of the stations requires 

adjustment. The long-term station at Fairport has been experiencing significant local subsidence 

due to nearby mining operations, and use of the data from this station has very limited use for 

lake-wide averages and datum control. It is recommended that the Fairport station be replaced 

with a new NWLON station between the existing stations at Fairport and Erie. This would also 

help to spatially resolve the lake level differences between existing stations at Fairport and Erie 

by providing another data point. Since lake level parameters vary in a longitudinal direction in 

Lake Erie, this new location would assist in further understanding the lake level differences 

between Port Stanley and Port Colborne along the Canadian shore. 

Lake Ontario 

The existing NWLON network in Lake Ontario has an adequate number and distribution of 

stations to meet requirements. Lake Ontario water levels are regulated, and there is significant 

overlap in the existing network for short-term mean lake level determination. The northern shore 

station pairs have the largest level differences and lowest correlations, but these are not large 

enough to recommend an additional station. The existing network size is still required to monitor 

the GIA in the lake basin and the distribution of the interpolated HCs for the western one-third of 

the lake are very complex and not well understood. 

Connecting Waterways 

Assessment of the observing networks in connecting waterways, especially for the upper Great 

Lakes, has been the subject of increased studies due to the recent extended period of significantly 

below normal lake levels. While there is apparent adequate density of water level stations 

(NWLON, Canadian, and USACE networks) for operational and regulatory purposes, there are 

gaps in information related to waterway flows, collection of which would require the installation 
67 

of current meters and flow meters. Just as for each lake basin (except for Lake Michigan), the 

total water level station network is a shared responsibility with Canada in most of the connecting 

waterways. The requirements for the stations in the connecting waterways are very different 

from the networks in each lake basin, but an engineering/hydraulic analysis is beyond the scope 

of this report. 

Monitoring Effects of GIA 

The impact of differential vertical land velocities on the hydraulic characteristics of the lakes and 

on the update of vertical reference systems will continue to be of significant concern. The 

correlation of seven-year mean water level differences between station pairs with projected 

differences due to differential vertical land movement underscores this and justifies the urgency 

for planning an IGLD update. Figure 54 compares the projected elevation differences from 

vertical velocities with the calculated mean differences for all of the station pairs in each of the 

lakes (excluding Lake St. Clair and the interconnecting waterways) and shows a fairly strong 

linear relationship. 

Figure 54. Relationship of projected elevation differences from vertical velocities to calculated mean elevation 

differences 2005 – 2011 for station pairs in the five major lakes.

The ongoing upgrade of the observing system in the Great Lakes with repeat GPS surveys and 

the co-location of CORS with water level stations should continue. Some of the CORS stations 

are just now accruing long enough time series to determine accurate estimates of vertical land 

velocities. A comprehensive analysis of the water level data, the GPS survey data, and the CORS 

data need to be undertaken to inform the GIA/PGR model and to better understand the 

differential rates and their impacts. An improved model will also better determine the location of 

gaps in the geodetic and water level observing systems. As these geodetic research and observing 

systems come on line, continuation of the existing network configuration on the lakes is vitally 

important for monitoring water level differences and relationships that contribute to the 

understanding the impacts of GIA here and elsewhere in the Great Lakes. 
68 

Network Operations 

The Great Lakes system is a complex interaction of hydrological, climatological, geological, 

hydraulic, and human-induced forces. The need for long-term monitoring of lake levels, flows, 

meteorological parameters, and vertical land motion is critical to understanding the system, 

managing the resources, and using the system strategically and safely for sustainable economies 

and coastal communities. Operation of such a network must be sustained and upgraded over time 

to meet user demands. Investment in station configuration with annual inspections, infrastructure 

upgrade, new technology, backup systems, and active applied research and product development 

are all required over the long term. Large portions of the Great Lakes system are covered by a 

bare-bones network, and loss of any one station(s) operation will put the required water level 

network for a given entire lake or connecting channel at risk and impairs the ability to manage 

the Great Lakes. Strengthening each station to ensure continuous operation helps mitigate this 

risk. 

The uncertainty in the interpolation of hydraulic correctors on each lake has highlighted the need 

for an updated seasonal gauging program in which additional locations are occupied for short 

durations and IGLD elevations are determined to supplement the permanent gauge networks. 

The last determination of this seasonal gauging program was for the IGLD 1955 update. The lack 

of precise updated datum elevations at these additional ports and harbors remains a significant 

gap. 
69 

ACKNOWLEDGEMENTS 

The author would like to acknowledge the technical reviews of draft versions of this report by 

several subject matter experts, who have significantly improved the content and technical clarity 

from earlier drafts. These reviewers include scientists and engineers from the NOAA/NOS 

Center for Operational Oceanographic Products and Services; from Environment Canada; from 

the U.S. Army Corps of Engineers, Detroit District; and from the NOAA Great Lakes 

Environmental Research Laboratory. Assistance is also very much appreciated from Helen 

Worthington in final technical editing and review and from Brenda Via in report production and 

publication. 
70 
71 

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14. IUGLSB, 2009. Impacts on Upper Great Lakes Water Levels, International Upper Great Lakes

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75 

APPENDIX 1. Hourly Water Levels for each Great Lake from 

Calendar Year 2005 

Note: the standard deviations used here are the standard deviation around the hourly mean 

obtained from averaging over the number of stations used for each lake. 
76 
77 
78 
79 

81 

LIST OF ACRONYMS AND ABBREVIATIONS 

BOM beginning of the month 

cm centimeters 

CO-OPS Center for Operational Oceanographic Products and Services 

CORS Continually Operating Reference Stations 

EOF Empirical Orthogonal Function 

ERS Ellipsoidally Referenced Hydrographic Surveys 

GIA Glacial Isostatic Adjustment 

GLERL Great Lakes Environmental Research Laboratory 

GLOS Great Lakes Observing System 

GPS Global Positioning System 

HC Hydraulic Correctors 

in inches 

IGLD International Great Lakes Datum 

IJC International Joint Commission 

km kilometer

LWD Low Water Datum 

m meter

MWL Mean Water Level 

NAD North American Datum 

NAVD North American Vertical Datum 

NOAA National Oceanic and Atmospheric Administration 

NOS National Ocean Service 

NWLON National Water Level Observation Network 

PBM Primary Bench Mark 

PGR Post-Glacial Rebound 

USACE United States Army Corps of Engineers