Find the oldest published version of the myth that George Washington threw a "silver dollar" over some river and see how that story called the coin. It must be understood that Washington supposedly did this as a young man, and that the USA did not strike "silver dollars" until 1794. However, Spanish-American 8 Reales were common in America and could plausibly have been called "silver dollars."
Being as the Potomac River is about a mile wide it was more likely the Rappahannock River. When George Washington was young the US Mint was not producing silver dollars. And this is only a legend as there are no facts to support it. If he in fact did throw anything across the river it could have been several foreign coins that resembled our Silver dollar or for that matter it could’ve been a piece of slate.