(River Karst isn't a spell.)
River Karst enters the battlefield tapped.
, : Tap target artifact with converted mana cost X or less.
: Add or to your mana pool.
Sacrifice River Karst: Add or to your mana pool.
, 
: Tap target artifact with converted mana cost X or less.: Add 
or 
to your mana pool. or to your mana pool.
