Mayflies are important indicators species of stream health. Some mayfly species are very sensitive to pollution, heat, low oxygen and other insults the environmental health of the water they inhabit. It is not surprising that mayflies are used as general indicators of stream health. The sudden disappearance of mayflies is likely due to some change in the water quality.
The sensitivity of mayflies can be used to judge changes in stream health over time. If mayflies were collected at a site 50 or a 100 years ago, but are no longer found at that site, it is an indication that water quality has changed over time. In 2009, the US EPA instituted a requirement for mayfly protection before surface mining permits would be issued.
Mountain top mining is controversial. Tons of rock is removed from the tops of mountains. Debris often covers streams and fills low lying areas. By 2005, as more than 1,200 miles of creeks and streams in Appalachia had been buried by mining debris. The debris can contain minerals that dissolve in water and are carried downstream. In part, the effects on mayflies downstream is the result of salt from the debris increasing the salinity of the water.
The EPA ruling has delayed issuance of dozens of permits. The coal industry is challenging the use of mayflies as an indicator species. They argue that mayflies are not present in the areas where the debris is dumped. It is not clear that they are arguing that the debris is not having effects downstream.
Mayflies are the “canary in the coal mine” of water quality. It is desirable to have cheap energy, but the external cost and damage to our environment must be considered.
Mayfly
Living With Insects Blog 
Someone says that Mayfly don’t eating when they are an adult. Because they just have 1 day life(when they are become an adult). That’s true or not?
Yes. Mayflies do all their feeding as larvae and adults mate and die as a final phase.