what type of pump do I need to pump water from a river to a pool about 300 feet?

Since this is a 1 time thing, why not rent one. Nearly any type of water pump will work, but use a screen on the intake to keep it clean.

If you have a house with a basement its a good idea to have a back up submersible pump around in case of flooding. You can go to any home store and purchase a submersible pump for about $70. The output of the pump will attach to a regular garden hose.

for filling your pool tape a scrap of window screen to the bottom intake so you don't suck up any critters and keep the pump above the bottom of the river. If you can wade in I'd just drive a stake into the bottom and then tape the cord of the pump to the top of the stake to hold it in place.

you could also make a more permanent mount for the pump and use it for watering your lawn to cut down on water and sewer charges. the river water will also be better for your plants than city water.

rent a trash pump and hose. will fill your pool in short order. Just place the suction hose in a plastic bucket. If not will get clogged and pump trash in your pool.

below is what the pumps look like. rental centers usually rent them for little money.

I like this one. The website has a product expert available with phone # in the right hand corner of the page, I would call and ask them. Not a bad price either if it will handle the 300feet (you didn't say if it was uphill) and ask how many gpm it is (gallons per minute).