They could still find stone in less monumental form. There are identifiable stone tool workings that are more than two million years old. Mind you, flint is fairly hard stone. Marble statues would erode if left exposed, but could be preserved for quite a long time under the right conditions. For that matter, good concrete can last a very long time. Maybe not 10,000 years, but we really can't be sure, since we haven't actually left concrete out for 10,000 years yet. Estimated time for various plastics to decompose varies widely between types. Some may outlast 10,000 years by quite a bit, although it might become unrecognizable to casual examination much sooner. Glass can last a very long time; glass relics are the modern equivalent of historical stone as a commonly-used but highly-durable material. After 10,000 years, there might be little of man left on the surface, but study in more depth and it will take far longer not to turn up clear signs that somebody was here and making a mess.
Along the lines of the dam... I was thinking of something slightly different. Maybe an ancient culture built a very long, curving (but only 8' tall) wall across a shallow valley, where there was only a river before. Then they went and died, and geological changes greatly increased the flow of water to the region, creating your shallow lake. The uniform edge of the wall means that water flows over it uniformly. Maybe it was some kind of paved "highway" that cut across the valley instead of a wall. Have it be made of some resistant material like non-rusting metal or glass. The wall could have been artistic, not practical, or maybe it was some other sort of structure that takes a wall form... like an aqueduct? Maybe it was some kind of dam, but only to create a shallowly flooded area, so that they could cultivate rice? Now unregulated and sealed up, the water just flows over instead. Depending on how old the culture is, there are a variety of materials that could feasibly survive. Especially if "modern" people came to value the falls aesthetically, and repaired the rim occasionally.
When they said "no trace" of human kind visible after 10,000 years, I'm sure they were referring to visible structures. They themselves said that the neat rows of gold bricks in the Federal reserves and Fort Knox would be there indefinitely since gold doesn't corrode. My comment was meant as support to the idea that a structure like Hoover Dam might indeed be a long lasting mechanism for a very wide waterfall.
Originally Posted by rdanhenry
Could such a waterfall be build? if so they could be the remains of an ancient dam type thing