1. rocks found in a river bed would typically be what type of rocks?

sedimentary
metamorphic
igneous
lava

2. parasitism is when

one organism benefits from another without it being affected
both organisms benefit from a relationship
one organism feeds off another making it weak or killing it
both organisms are hurt from a relationship

 Igneous or metamorphic. Depending on the energy of the river, they could be anything from large, rounded cobbles or pebbles (as per classification of the Udden-Wentworth Scale), or sand and gravel as identified under other systems. Or angular clasts that haven't been rounded by moving along the bed. Note that the Udden-Wentworth is usually applied to sedimentary features, but can also be used in classifying the *size* of any rock. 

3. is true i think i should be right