Since "1 acre-foot of water ( *1 ) equals 1 U.S. survey acre-foot =
43,560 U.S. survey cubic feet
≈ 1233.4892384681 m3
≈ 271,329.700571 imp gal
≈ 325,853.383688 U.S. gal[nb 1]",
lets call it 325.9K gallons, a (surface) 50,000 acre lake rising 1 foot would be adding 50,000 X 325,853.383 = 16,292,669,150 (US) gallons of water.
Roughly 16.3 Billion (US) gallons of water per foot of depth in that 50Kf² lake. https://en.wikipedia.org/wiki/Acre-footHowever, as noted by others, most bodies of water expand in area (*2) as the water rises, so the number of acre-feet and gallons are also increasing with each 1 foot rise in lake level.
Similarly, most don't realize just how much water, as vapor, clouds hold, even puffy little cumulus clouds hold many thousands of gallons of water.
Down here near the Gulf, one can actually sometimes see those billions of gallons of moisture being transported north and northeastward as clouds that eventually drop as (often) major rainfall/snowfall events in the more northern states. It often takes the form of a certain type of dense rapidly moving cumulus streaming n/ne, when I see them I'm pretty certain somebody far away is going to get clobbered with precip. These clouds actually appear a bit 'tropical', like those associated with TrSm/hurricanes (or feeders to TS/squall/frontal systems), but without the large-scale rotation. Convection, it's a bear.
*1 "As the name suggests, an acre-foot is defined as the volume of one acre of surface area to a depth of one foot.
Since an acre is defined as a chain by a furlong (i.e. 66 ft × 660 ft or 20.12 m × 201.17 m), an acre-foot is 43,560 cubic feet (1,233 m3)."
*2 I live fairly high above the San Antonio River surface on a sort of ridge, normally the river in it's valley is just visible as a mid-distant (~<1 mi) treeline, and the river is only ~<25 yards wide.
In the 'great floods' of 1998 and 2000, the river flooded to the point where it was two or three miles wide (treeline was definitely submerged) , an awesome sight (took a bunch of pictures). The river waters inundated many hundreds of thousands of acres of land down to the SA Bay. It came up and covered the US highway below my place (still ~100 feet downhill), cutting it off and forcing a detour of many miles for NW/SE traffic towards SA. The ex's place is at much lower elevation, and the river's edge came within a few hundred yards of it (some people have since built homes down there...suspect that they'll be surprised to have waterfront property the next time).
