PIE *ser - to flow

Hello,

Could you post some reliable words that belongs to the PIE *ser - to
flow ?
Finally, I could find only Latin serum here.

Thanks,
Marius

P>S> Additional question: Is the PIE *sreu and extension of ser? If
yes, how that extension was formed :
- as *sr-eu? or as *ser-u > *sre-u ?