HOUSTON — Hurricane Harvey has released 14 to 15 trillion gallons of water on top of Houston since last Friday. And though the storm has left the city, the catastrophic flooding continues.

Scientific American reports that despite being flat and prone to flooding, Houston lacks a high-tech drainage system, and relies on slow-moving creeks or streams called bayous to empty its flood waters.

The city's 10 bayous typically flow from west to east, before going through the Houston Ship Channel into Galveston Bay in the Gulf of Mexico.

The Addicks and Barker reservoirs were built in the 40s to help contain floodwater, but the Addicks has been pushed past capacity, and the Barker might soon overflow as well.

Given the speed and volume of the rainfall, surface soils become saturated quickly, and are soon too full to take in any more water. This is made more difficult by the less-absorbent concrete and asphalt that cover the city.

Water cannot evaporate quickly, either, given Houston's humid climate. And with slowly receding floods already soaking through items and property, the damage may be long-lasting.

No one currently knows how long it will take for the floods to drain completely, with experts estimating it will take anywhere from a few days to possibly even weeks.