Red sediment on my pool cover- What is it

[SMB_Pool_Idiot]

I get this stuff every year. I am getting ready to open the pool for the season and I have this reddish-brown fine sediment to deal with. Some of it will probably end up in the pool when removing the cover. Just wondering what it is and what causes it.

 

[kul]

Weird. Could it be coming from the trees? What does it feel like? Is it sand?