Creating a Lake with a Dam.

Create an area on your terrain to hold a lake. 
If you want a dam with a river flowing from it, pick a place for the lake that has good drainage on one side.

Now, you can create a plane and size and move it to where it shows how a level spot will look, so that you can modify the terrain to get what you want using the Unity terrain tools.

You can show and hide the plane as you work on the area.

Next, create a river with only 2 Nodes/Points with the start node at the far end (2nd node near the dam, if you plan to make one). Use the "Smooth" spline button whenever you move a Node/Point.

You may need to move the nodes to get the river/lake level and flowing the right direction.  The nodes should be near the same height.
You can modify the terrain under each node to get the correct water level, along with the "Start Depth" value.  Adjust the River Width, Vertex Distance and Bank Overstep to cover the area.  If the bank doesn't meet up with the edge of the lake, you may need to modify the terrain slightly.

Next, place a Dam model or use the new Extrusion Pro Extension of GeNa Pro to create a dam.

Make adjustments to the lake spline River Extension for the Start/End Cap Distance to keep from pushing through the Dam model and to make sure it reaches the back edge of the Dam.

Then, create a new GeNa Pro Spline and add a River Extension to create the river flow that comes from the Dam.
 

Note that, because we used 2 separate splines for the Lake and the River, we can place them at different heights.

That is it for this tutorial. If you have any questions on the process that I did not cover for your needs, please feel free to ask.