Permanent streams flow all year round usually by relying on input from groundwater sources. When a stream flows at normal discharge, this is called ‘base flow’. When runoff flows into streams and it rises to full capacity, this is called ‘bankfull capacity’. When the streams’ flow exceeds the bankfull capacity, this is termed a ‘flood’.  The stream's water spills over into the river’s ‘floodplain’. Flooding occurs regularly in non-managed streams and provides an important connection between the river and its floodplain habitats. The floodplain is a very productive area, and provides nutrients to the river. Small wetlands and backwaters are also found in floodplains, and these often provide habitat for amphibians and fish to lay their eggs. When the river spills into its floodplain, the fish eggs that have hatched and matured are released back into the river to live. There are 419 miles of permanent streams in the Sac River Basin.

Not all streams flow all year long. These are called “intermittent streams”. They are usually streams of a lower order (smaller), and they are found at higher elevations within the watershed. Because of their elevation, they are not able to rely on groundwater sources to keep flowing.  Within the Sac River Basin there are 173 miles of intermittent streams. Some of these, while not flowing, maintain permanent pools of water along their length.