WATER DENSITY AND ICE
Unlike most substances on Earth, the solid form of water is less dense than the liquid form – in other words, ice floats. If ice didn’t float, our oceans would have filled up with ice billions of years ago and life on Earth, if it even existed, would look very, very different.
The annual heating and mixing cycle of deep lakes results in layers of different density, especially in the late summer and in ice covered winter lakes. When a lake is stratified by density, the deep layers are isolated from the atmosphere and may be in waters too dark to support photosynthesis. Dissolved oxygen consumed in this deepest layer (for example from decomposition of dead plant material) cannot be replaced by mixing with the atmosphere or through oxygen formation during plant photosynthesis.
Stream flow, or discharge, is the amount of water that flows past a specific point in a stream over a specific period of time. The two components of stream flow– velocity (how fast the water is moving) and volume (the amount of water in the stream) combine to determine the energy of the water. A water’s energy greatly affects the shape of the stream as well as its biological and chemical characteristics.
Discharge is not to be confused with velocity, however. Velocity refers to how fast the water is moving, whereas discharge tells us how much water is moving past a certain point. For this reason, discharge is the measurement water quality managers use to assess how much of something (such as pollutants) are flowing past a given point in a river. We can think of discharge as a “box” of water.
Among other factors, groundwater has a big impact on stream flow, especially during drier seasons. Groundwater seeps out of springs in the stream bed, maintaining flows and keeping stream temperatures cool enough to support aquatic life. Excessive withdrawals of groundwater often cause nearby streams to run dry.
Low flows often correspond to higher stream temperatures and concentrate pollutants: maintaining adequate stream flows during summer is crucial to ensure the health of our streams.
Measuring stream flow
To calculate stream flow you will need to determine average velocity (measured in feet per second – ft/s) and the average area of the cross-section of the stream (measured in square feet – ft2). Multiply velocity and area to find stream flow. Stream flow is measured in cubic feet per second (cfs). Stream flow = velocity (ft/sec) X area (ft2) = ft³/sec (cfs)
Velocity is determined by timing how long it takes an object (something such as a ping pong ball) to travel 50 feet along your stream section.
Area is determined by measuring the cross-sectional area of the stream (width multiplied by average depth).
USGS real time stream flow -You can compare your results of calculated stream flow with real time data.
USGS Water Science School: Water reaches its maximum density at 4.0 °C. Click here to learn about other cool water properties.