Surface water is freshwater we see above ground. It includes streams, lakes, ponds and wetlands. Groundwater is freshwater that lies beneath the earth's surface. Most groundwater is stored in aquifers. Aquifers are areas underground where rock and soil are very porous.

Many features of streams and rivers undergo constant change. Throughout a single day we may observe changes in water level, salinity, water flow speed, discharge (the amount of water passing a point on the river in a given amount of time), and sediment load (the amount of soil and gravel suspended in the water). These changes are caused by water input into the river. Rain and melting snow increase the amount of freshwater in rivers. Rain falls directly into rivers, and rainwater and melted snow on land surrounding the river runs downhill into the river.