The amount of water flowing in rivers throughout Washington state is well below normal for this time of year. 

Many of those rivers are in drought-affected areas, according to data from USGS

Using a seven-day average, the Department of Ecology found more than half the rivers in the state are showing streamflow in the bottom 10th percentile of flows measured for this time of year. Streamflow less than the 25th percentile is considered "below normal." 

A handful of those rivers are experiencing the lowest streamflows ever as of Thursday, according to USGS. 

"Low streamflows are the most visible evidence of drought. In Washington, our streams are fed by snowmelt and rain and when one or both is lacking, our streamflow’s respond accordingly," said Washington State Department of Ecology Drought Coordinator Jeff Marti.  

WATCH: Washington's drought concerns explained

Rivers are "invaluable," the USGS explains. They are used for drinking-water supplies, irrigation water, producing electricity, flushing out treated waste, and more.

Marti said low streamflows mean less living space for fish and other aquatic species, which could make them more vulnerable to predators and even become stranded. 

"For larger fish, it can become difficult or impossible to move up and down a river. All this reduces the ability of a fish stock to reproduce and replenish itself, which can affect population numbers many years into the future," said Marti. 

Low streamflow can also impact water needed for drinking and irrigation. 

"Even large cities that store water count on the return of fall rains to refill their reservoirs. In years like this year, they may need to use more of their stored water than normal because nature isn’t providing as much," said Marti. 

It's no surprise that streamflow in rivers throughout western and central Washington is below normal. As of Tuesday, much of the state was either abnormally dry or in a sever drought, according to Drought Monitor. 

The Puget Sound area, which has not been declared a drought-affected area, is also seeing very low streamflow.