LEARN
We believe everyone should understand the waterways around them. That’s why RiverAware makes river data simple and easy to monitor, empowering you to make informed decisions and protect your peace of mind.
Explore the science behind our app.
Select a term below to learn more.
River Hydrology 101
Measuring Rivers
Flood Terminology
Hydrology
The study of water—where it comes from, how it moves, and how it affects the Earth. Scientists use hydrology to understand floods, droughts, and how to manage water for drinking, farming, and other uses.
Watershed
An area of land whose water drains into one common body of water, such as a river, lake, or ocean.
Diagram of a watershed showing how precipitation collects into a single body of water (Seacoast Science Center)
River
A large flow of water that move across land. Rivers usually start in the mountains or hills and flow fresh water into a lake, sea, or ocean. Rivers are important because they bring water to places where people and animals live, and they help move things like soil and plants from one place to another. Rivers can flood when a watershed receives large amounts of rainfall or snowmelt in a short period of time.
River Zones
A section of river that has different characteristics based on where is falls along the river system. Rivers are usually divided into zones from where they begin (source) to where they end (mouth). Zones differ by how fast the water moves, how wide the river is, and how it shapes the land. Each zone plays an important role in how water, plants, animals, and landforms interact. Read below to learn more about each type of river zone.
Diagram showing river zones along a river system (Adapted from a resource kit released by the HI-AWARE Consortium)
Source Zone
This is where a river begins, usually in the mountains or from a spring, glacier, or rainfall. The water here moves fast, is cold and clear, and carves through rocks to create rapids and waterfalls. Rocks and soil from the source zone flow downhill in the river waters.
Transition Zone
In this zone, the river slows down and widens as it moves from the mountains to flatter land. The water is warmer and carries small pieces of dirt and sand, creating nutrient-rich soil that is good for farming.
Floodplain Zone
This is the lowest and widest part of the river, where water flows slowly through flat land. This zone is prone to flooding during heavy rain and snow melt, spreading nutrient-rich soil that helps plants grow. It often ends at a delta in a lake, sea, or ocean.
River Capacity
The maximum amount of water a river can carry before it overflows its banks, otherwise known as flooding. It is often determined by the shape and slope of the river, along with the characteristics of the surrounding land.
Streamgaging Station
A device near a river or stream that collects and reports various types of river data, such as how high the water is (stage) and how fast it’s flowing (discharge). This data helps scientists and communities monitor river levels, predict floods, and manage water resources. Most streamgages in the U.S. are run by the U.S. Geological Survey (USGS), and apps like RiverAware use this information to keep people updated about rivers nearby.
Diagram of streamgage that uses pressurized gas to measure the stage of the river (USGS)
River Stage / Height
The height of a river’s water above a reference point, measured in feet. Sometimes referred to as “river level”.
River Discharge / Flow
How much water is flowing in a river and how quickly, measured in cubic feet per second.
Diagram showing how discharge is measured in a small stream (USGS)
Hydrograph
A chart showing how river levels change over time. Hydrographs help people understand rivers and predict floods by showing:
When the river is rising (meaning more water is flowing in).
How fast the river is rising (which can tell us if a flood might happen soon).
When the river is going back down (showing when the flood is over).
Hydrographs are essential for monitoring and understanding rivers, analyzing flood risks, and municipal water management. Apps like RiverAware provide accurate, real-time hydrograph data from streamgaging stations across the United States.
Example of a hydrograph showing moderate flood stage levels in the RiverAware app
How to Use Hydrographs During Floods
You read a hydrograph as a line graph, with time on the bottom (X-axis) and water height or flow on the side (Y-axis). The line represents the current water level at the location of that streamgaging station.
Look at the line: A rising line means water levels are increasing, and a falling line means they are going down.
Check the flood stage: If the line is near or above the colored flood stage indicators, the river could flood soon.
Watch the trend: If the water level is rising quickly, it could mean a flash flood is coming.
Set up alerts: RiverAware lets you set up custom alerts in the event that water levels reach a certain height, keeping you aware of impending floods.
Baseflow
Baseflow is the normal, steady flow of a river that comes from underground water sources like springs and groundwater. Even when it hasn’t rained for a while, baseflow usually keeps a river from drying up. It is the lowest water level a river usually has.
Peak Flow
Peak flow is the highest water level or flow in a river over a certain time period, usually after heavy rain or snowmelt. It shows the point when the river is carrying the most water and is most likely to flood. If a river enters flood stage, the peak flow is the maximum amount of water moving through the river before it starts to recede. After peak flow, water levels gradually drop unless another storm adds more water.
Floods & Flash Floods
A flood is when water from a river, heavy rain, or melting snow covers land that is usually dry. Floods cause water to flow into roads, fields, and homes and are sometimes life threatening. Areas near large rivers or in wetter climates can experience regular flooding events. Most floods are the result of a storm of some kind.
Flash floods occur very quickly after heavy rain, often in just a few minutes or hours. It is a sudden, fast-moving flood that can be very dangerous because it gives people little time to prepare or escape. Flash floods are more common in areas with a dry climate and rocky terrain because the type of soil there does not absorb water quickly, so excess water flows over the land instead.
Flood Advisory
When there is risk for flood, alerts are made available to the public according to the urgency and potential for dangerous conditions. A flood advisory is issued while heavy rain is falling to areas that typically experience small flood events, such as urban areas with poor drainage, construction zones, or near small creeks and streams.
What to Do
Be aware of your surroundings. Avoid flooded areas and roadways. Never drive into flood waters, even if they appear shallow.
Flood Watch
A flood watch is issued when heavy rain is in the forecast and the area is expecting flood conditions, but is not currently flooding. Flood watches are issued for large areas, such as entire counties, and are often in effect for multiple days.
What to Do
Prepare for flooding and avoid travel. Monitor weather forecasts closely and make sure neighbors are aware of upcoming flood potential. You can use apps like RiverAware to make flood monitoring easy, and it lets you share updates and river data directly from the app.
Flood Warning
If a flood warning is issued, it means flooding is imminent or currently happening. There may be a rapid rise in fast moving waters, vehicles could be swept off of roadways, and homes can be damaged. Flood warnings are an emergency and are only in affect while a flooding event is active.
What to Do
Immediately take action to protect yourself and your loved ones. Seek higher ground if located in a floodplain, otherwise do not travel. Avoid water-covered areas and roads. Monitor weather conditions closely with information from a trusted source, such as RiverAware.
Flood Stage
Flood stage refers to the water level that could cause damage to people, property, or businesses. Flood severity varies along a river because of different land features. The National Weather Service (NWS) typically only calculates flood stages for rivers near populated areas. If you live in a rural community, the river stations near you may not have defined flood stages.
Action Stage
This is when you need to take steps to prepare for or reduce the risk of flooding. People living in affected areas should closely monitor water levels when the stage reaches action level or above.
Minor Flood Stage
Flooding that causes little or no damage to property but may still pose some risk to the public.
Moderate Flood Stage
Some structures and roads may be flooded. People may need to evacuate or move property to higher ground.
Major Flood Stage
Large areas of land, including structures and roads, are flooded. There may be significant evacuations and moving property to higher ground.
Record Flooding
Flooding that matches or surpasses the highest level ever recorded at a location. Each year, we see more cases of record flooding.
RiverAware lets you monitor river conditions, get real-time flood alerts, and share river data instantly. Download the free app today to experience the new generation of flood preparedness.
Thirsty for More?
Check out the resources below to learn more about hydrology, streamgaging, and the science of flooding.
Hydrology Education from the National Weather Service (NWS)
Water Science School & Streamgaging Basics from the U.S. Geological Survey (USGS)
Severe Weather 101: Flood Basics from the National Oceanic and Atmospheric Association (NOAA)
Climate Change Indicators: River Flooding from the U.S. Environmental Protection Agency (EPA)
A Glossary of Flood Terms from the Georgia Department of Natural Resources