Not sure about what sorts of things are currently being done, but major modifications had to be after since the dam was built, mainly to improve the interface between the dam and the surrounding topology. Structural integrity of the dam is impacted be water leaking around the sides of the dam and more critically be water leaking under the dam, which leads to a phenomenon called uplift. Eventually tectonic activity in the area will cause the failure of this interface and the dam will fail (unless as another poster has mentioned the Colorado River dries up enough that Lake Mead drops to a very low level.
That said, even if the dam fails there will be one or more honking big chunks of concrete. Eventually the river will erode these away (it’s done a pretty good job of erosion a bit upstream from the dam) but that would be more on a geological time scale.
The Glen Canyon Dam might be a good candidate. Upstream from the Grand Canyon, another arched concrete gravity dam. Actually used more concrete then Hoover Dam (5.4 million cubic yards vs. 4.4 million), but its profile is thinner and the surrounding geology is sandstone rather than granite. But still a honking big chuck of concrete in the desert.
The Owyhee dam in (dry) SE Oregon was the “beta” dam for Hoover. Same design, construction methods, just smaller. So if Hoover will last 10,000 years Owyhee should last … 9,000 years? Oops, make that 8,996 years due to being 4 years older.
As part of the Grand Coulee project in E. Washington an artificial holding lake was built using two dams across a dry coulee. Water is pumped into the lake. If the pumping stops, the lake will probably eventually dry up (mainly due to seepage) and the two dams will just sit there. Wind erosion is going to take a long time.
There’s a floating barrier to keep the boaters away from that end of the reservoir. No doubt it’s not enough to keep the determined “Don’t worry, I know what I’m doing.” folks out.
I’ve heard tales of a guy falling into the Owyhee Project siphon* across the valley lower down, but none about the glory hole.
That’s fair enough. Humans modified the floor at Meadowcroft, but it’s definitely a natural structure, not human-built.
There are, of course, many examples of human-built underground structures, considerable traces of which are likely to last a lot more than 10,000 years.
Here’s a site that lists seven of these, including one that’s already around 5000 years old.
More substantial than any of those is the Cheyenne Mountain complex built to withstand nearby large-yield nuclear detonations. Traces of this will probably persist at least several million years after humans stop paying attention to it.
