Find-A-Feature Challenge

Find-A-Feature: Water Clarity

Welcome to Find-A-Feature Photo Challenge! Periodically, we will showcase a new geological or ecological feature and challenge you to find something similar in your neighborhood. Science is everywhere - take a look around! Show us what you see! Tag us on Twitter or Instagram (public accounts only) @USGS_YES or send your pic to

What is water clarity? Water clarity, most simply stated, is a measurement of how “see through” water is. But, there’s a lot more to it than that. Many factors can influence water clarity, such as the turbidity, algae abundance, the presence of pollutants, and more. Water clarity generally decreases as the suspended load increases. Muddy water is harder to see through. Learn all about water clarity here on the water clarity and turbidity section of the USGS Water Science School.

How is water clarity measured? One simple tool used to measure water clarity is called a Secchi disc, which is a weighted disk with white and black quarters on the top and a measuring tape or wire attachment to measure the depth at which the disk can no longer been seen. Want to make your own Secchi disk? You can find examples of how to make your own with the GLOBE Secchi disc or SERC Secchi disc, or for a super easy DIY Secchi disk, you can even glue 2-3 old CDs together (one alone will not sink) and use paint or nail polish for the black and white sections and a measuring tape or metal yard stick. Learn some basic water sampling techniques that you can do near your own house, and what you can learn about the water you collect. Learn more about current and past USGS research projects on water clarity here.

What about water color? We have all seen different colors of water – but why? Even pure water is rarely colorless. Sediment can make water look brown, organic material and algae can make the water look green, dissolved iron casts a brownish-reddish hue, and water with a lot of air in it can give your water a milky white appearance. Regions with low algae and white carbonate sand – think about a tropical beach – often make the water look turquoise, and glacial water often looks bright blue due to the high amount of suspended glacial flour (fine-grained glacial sediment). Water depth also plays a role. Deep lakes, such as Oregon’s Crater Lake, often impart a very dark blue color. For a great link about water color, visit this page of the USGS Water Science School.

Resources for teachers:

USGS Water Science School (WSS):

USGS Water Resources Mission Area: