Which way did Warrego flow? Some people were a little confused as to whether Warrego Valles consisted of a bunch of small rivers that eventually joined together into a larger one, or one large river that split apart. If we look at the region, we see that one end of the Warrego river system was in the uplands, and the other end is in the plains. Since water flows downhill, this means that the water probably flowed from the uplands to the plains (from north to south)—meaning Warrego consisted of smaller rivers that joined together.
How old (relative to the plains and/or uplands) is Warrego Valles? Using the cratercounting method, most people determined that the upland region is older than the plains. Since Warrego is located in the uplands, many people said that it is therefore older than the plains. This is not necessarily true. The Warrego river valley system is on top of the uplands, so it is younger than the uplands. As for whether Warrego is older or younger than the plains, that's harder to tell. By the looks of things, it might even have formed very recently. All we know is that Warrego is younger than the uplands.
You could go a little further, though, and examine how many craters are on top of the valley system: only a couple tiny ones. This is vaguely about the same amount of craters you'd see in the plains, so Warrego might be about the same age as the plains.
Where did all the water go? The first TakeHome Question addressed the fact that we see all kinds of features that look like they were made by water, but we don't see water on Mars today. If there was a lot of water on Mars in the past, where did it go?
Just saying that it evaporated isn't good enough. Water that evaporates becomes water vapor, so we'd expect to see a large amount of water vapor in Mars' atmosphere. We don't.
There are two main theories for what happened to the water. One is that some or all of the water seeped underground and now resides there in a frozen layer beneath the Martian surface.
The other theory has the water molecules broken apart by ultraviolet radiation. Since Mars has a thin atmosphere, more UV can get in, and it can have high enough energy to break the H2O's apart into H's and O's (hydrogen and oxygen). Hydrogen is very light, so by colliding enough with the other molecules in the atmosphere, it is able to quickly get to fast enough speeds to escape out into space. The oxygen is too heavy to easily escape, but it is very reactive, and it combined with minerals on the ground. This is why we see so much iron oxide (rust) on the surface today. Water features in the north? When we compare the northern and southern hemispheres of Mars, we see that the northern hemisphere is generally a lot lower and smoother. We could imagine that if there were oceans on Mars (possible, considering the amount of water needed to carve all of these riverbeds), most of the northern hemisphere would be ocean. And now things are starting to make sense: we see a lot of river valleys in the southern hemisphere that “flow” northward until they reach a spot near the equator, then stop. These locations must have been where the rivers reached the ocean. That there was a large ocean there explains why we don't see more river valleys in the north, or very many craters for that matter.