Background: Hydrology of the Great Lakes, Long-Term Trends

Water is in constant motion. This movement is called the hydrologic cycle and involves evaporation, condensation, precipitation, groundwater, transpiration and runoff. The hydrologic cycle of the Great Lakes basin determines water supplies to the lakes. Understanding the hydrologic cycle is an important part of understanding the Great Lakes ecosystem.

Hydrologic Cycle

  • Evaporation — Water from the surface of the earth (from rivers, lakes, seas and oceans) is transferred to the atmosphere, changing from a liquid to a gas. Lake evaporation typically has the greatest effect on water supplies during the winter months as dry air and warm water result in massive evaporation.
  • Condensation — As moist air is lifted into the atmosphere, it cools. This cold, moist air, called water vapor, condenses to form clouds.
  • Precipitation — Moisture moves around the earth in air currents, until it returns to the surface as precipitation (rain, snow). In the United States, the prevailing winds are usually from west to east.
  • Groundwater — Once precipitation reaches the ground, the water may filter into the ground and become groundwater. From there, water seeps into the Great Lakes, oceans, rivers and streams.
  • Transpiration — Water is carried from the ground through plant roots to small pores on the underside of leaves, where it changes to vapor and is released to the atmosphere. About 10 percent of the moisture found in the atmosphere is from water released by plants through transpiration.
  • Runoff — Water that is not absorbed into the ground and remains on the surface of the earth is runoff. The runoff water empties into rivers, streams and eventually the Great Lakes and oceans, where the cycle begins again. Runoff comprises a significant part of the Great Lakes water supply, particularly during the snowmelt season, late March through early June.

When discussing precipitation and runoff in your class, it’s helpful to first explain water flow and to describe your watershed. A watershed is an area of land that drains into a river system. Any water entering a watershed, usually as precipitation, travels from higher elevations to lower elevations. As the water moves downward, it forms streams and rivers. The channeling and pooling of water is determined by the shape or topography of the land.

Teacher Tip

One way to introduce the term watershed is to ask students to separate the word into “water” and “shed” (to pour or cause to pour off; to emit) and discuss what each word means.

It’s All Connected

Water continues to move downward, and rivers may join with larger lakes (e.g., the Great Lakes) or other larger rivers as they head toward the ocean.

The Great Lakes Water System

The diagram below shows how water flows through the connecting channels of the Great Lakes, through the Saint Lawrence Seaway and to the Atlantic Ocean.


Diagram courtesy of Michigan Sea Grant,

Lesson Resources:
• U.S. EPA, Natural Processes In the Great Lakes
• U.S. Geological Survey, The Water Cycle


Variables in the Data Sets

Water Surface Temperature:
The exact measurement of water surface temperature ranges from simple thermometer readings at specific depths to ultra-sophisticated satellite infrared radiometer indirect methods. Whatever method is used, the temperature of a lake over time can tell us many things. Living organisms and plants have preferred ranges of temperatures in which they thrive, grow and reproduce. Any alteration from this can affect their survivability and change the ecosystem where they grow.

A change in temperature will also have an effect on the water chemistry of a lake. Generally, when the temperature increases, the rate of chemical reactions also increases. This affects metabolic rates of cold-blooded animals, as well as growth and reproduction of simple organisms and plants. However, the warmer the water, the less oxygen it can hold. This in turn can affect the life in the lake.

Thermal Stratification:
Lakes are subject to thermal stratification, layers of water with different temperatures and different concentrations of oxygen. Cold water is denser than warm water, causing the cold water to sink. In warmer months, the surface layers of a lake are heated and the mixing of the atmospheric gases causes an increase in the water’s oxygen content. The lower layers are colder. As autumn approaches, the surface layers become cooler and, in turn sink, displacing the lower layers. The water turns over and mixes. This is called fall turnover. A similar process happens in the spring, referred to as spring turnover. Thermal pollution — an influx of hotter or colder water — from municipal and industrial discharges in urban areas can cause changes to water temperatures and oxygen content.

Monthly Mean Overland Air Temperature:
The monthly mean overland air temperature refers to the average of the air temperature over the land. The atmospheric temperature has a direct effect on the surface temperature of bodies of water. Examining these trends can provide clues as to the effects of global climate changes.

Monthly Evaporation:
The variation in the amounts of lake evaporation affects the heat content, or amount of heat energy stored, in bodies of water. Evaporation is one way that heat can be lost from lakes. Maximum evaporation occurs when the water temperature in the Great Lakes is much warmer than the air moving across them. This occurs particularly in the fall before the lakes have a chance to develop an ice cover.

Overlake Precipitation:
Overlake precipitation is rain that falls into the lake, which is very important to maintain water balance in the lakes. It is important that hydrologists track overlake precipitation in order to maintain proper water levels in reservoirs that serve communities. Climate change has contributed to increased variability of overlake precipitation, making tracking and water level management even more important. Overlake precipitation can be measured using various techniques from simple gauges to radar observations in populated areas.

Monthly Runoff to Lake from Land Surface:
Surface runoff is the water flow, which occurs when soil is saturated with water and the excess flows into the lake in this case (specific to the data set). The water can come from rain, snowmelt and drainage from industrial and municipal sources. The runoff can pick up various contaminants along the way, such as petroleum, pesticides and animal waste, are common contaminants in the Huron River watershed in Ann Arbor.

River Flows:
Measuring the flow of a river or stream is another aspect of not only water management but also hazard management, environmental research and infrastructure design. It is measured in four steps:

  1. Stream Stage — Measuring the height of the water surface at a location along the stream or river.
  2. Discharge Measurement — Tracking the quantity of water passing a location along a stream.
  3. Stage-discharge Relation — Defining the natural-but-often-changing relation between the stage and discharge.
  4. Converting Stage Information to Stream Flow Information — using the stage-discharge relation (developed in step 3) to convert the measured stage into estimates of water flow or discharge.

Diversions to Canals:
A diversion is any transfer of water across watershed boundaries through a man-made pipeline or canal. Diversions supply a steady stream of water for energy production, irrigation, commercial shipping, recreation, municipal water supplies and flood prevention. Diversions can transfer water in to or out of the Great Lakes basin. Due to the natural drainage of water from north to south in Lake Michigan, many canals are located in the Chicago area and help regulate water flows that could overtake the area.