River Placement Questions - Where can they begin?

[waldronate]

A river/stream/spring will start/occur any place where the water table is higher than the landscape. Water will continue to flow downhill as long as the water table is above the landscape, the water will flow downhill. There are lots of apparent exceptions, but those are usually related to changes in ground permeability (which affects the water table) more than any actual exceptions.

The most common places for the water table to be high are where there's lots of water, such as below snow caps on mountains. Mountains tend to accumulate more rainfall than plains, and rivers will often start there. However, if all of the water for a river were required to come from high sources, there would be very few rivers! What normally happens is that water falls all over land and soaks into the soil. That water forms the water table and tries to find a uniform level. When it impact the surface is when it becomes visible surface water. It's all of that water stored in the water table that makes rivers get larger as they flow along.

Apparently weird rivers happen a lot. A fairly common problem is disappearing rivers. Rivers flow along, collecting tributaries, and growing nicely. Then they hit a sand area, like an old dune field. The river shrinks dramatically, sometimes disappearing entirely. It may suddenly appear miles away from the sand. A river in karst terrain can just fall into a cave and be gone. It may appear later or it may not.

[Zedfelos]

Thanks for the replies, everyone!

So, what I'm getting is that as long as there is enough water accumulating in an area (for potentially multiple reasons) and that there is a place downhill for that water to go, a river can form. I especially like what you said about mapped rivers vs. unmapped rivers, rdanhenry. That really helps me to conceptualize what I'm mapping here.

Let's see what I can make of this!

[JoshStolarz]

Thanks for the replies, everyone!

So, what I'm getting is that as long as there is enough water accumulating in an area (for potentially multiple reasons) and that there is a place downhill for that water to go, a river can form. I especially like what you said about mapped rivers vs. unmapped rivers, rdanhenry. That really helps me to conceptualize what I'm mapping here.

Let's see what I can make of this!

There's already been some great answers to the question, but I will throw in a personal example regarding the idea of mapped vs. unmapped rivers. Here in Oregon where I live, there is no shortage of mountains and it rains all winter. Needless to say there are quite a few large rivers running through the state. But, I have seen several maps that actually do map all the tiny streams that flow down from the mountains and there are so many of them you can pretty much see the form of all the individual mountains. Basically, anywhere two mountains meet or there's a low point, there's a strong possibility there will be at least a seasonal stream that forms.

- Josh