Replies: @RobertLM78 @ulisse @JulioC Thank you all very much for your interest and your nice comments so far! @MissVanleider Thank you too! The bridges are from various sets; Pegasus' streams, diggis' streams, and mostly the Neko brooks. Contents: Please forgive me the crappy text design - sometimes the editor won't work right and doesn't allow me to use Enter and the arrow keys; I had to type this in Notepad and copy&paste it. In this special entry, I will explain the way I made the rivers in entry 5. It's nothing special, I think, and it has only been an experiment, so expect from me to change things occasionally, but since @Bobelboy asked me, I thought I could give an answer.
I'm demonstrating, as far as I can remember everything important (please don't hesitate to ask me, my memory doesn't seem to serve me well these days), how to make a river on this landscape. As you can see, I've already dragged some water MMP's there - this is usually the first step, I'm outlining future development with MMP's, especially settlement borders, future roads, and waterways, in case I can't develop them right away:
Step 1: Terraforming As you can see, we are starting in the mountains and then going down to flat land. I'm only depicting the mountain part; you can just apply the schemes to the flat land like that. You can also see that there's some terraforming error in the mountains; the river will go through a dump in the terrain. Occasionally, such terraforming errors are happening, thus, as a first step, I'm using street/road stubs to flatten and fine-tune the terrain a bit:
Step 2: Laying out the waterway Next step, but most important: Which stuff to use? I'm mostly using ploppable water instead of in-game water; it gives me more designing possibilities. I do have a lot of stuff in my Plugins folder; fortunately, a lot of it is really useful. Most frequently, I'm using Pegasus' Ponds/Streams and Terrain Kit, the RFR Plopwater System, Diggis water system, and the Neko Brooks:
For Pegasus' content, please search in the PLEX download subsection for his content.
For Diggis content, please make a power search for "diggis" over at the LEX.
Unfortunately, I don't have a link for Neko's brooks. They are on some Japanese website.
RFR Plopwater system: https://community.simtropolis.com/files/file/27199-rfr-plopwater-pack-1/
For waterfalls, I'm using my own waterfall lots: https://community.simtropolis.com/files/file/32036-small-ploppable-waterfalls/ And some pictures now. The RFR MMP's are very versatile, as you can see, there are pieces for both orthogonal and diagonal sections, and for water flowing slowly and rapidly, as well as for turbulences.
Next, some pictures of Pegasus' content, diggis' content, and the Neko brooks. I especially use these systems often, they are very versatile as well and come with some overhanging lots, as well as lots designed to fake a small bridge:
The 2nd piece of the RFR plopwater system is very useful to make transitions to Pegasus' streams and diggis' water. You can also use the Tahoe water by Pegasus to make transitions to the Neko Brooks, if you hide the sharp edges a bit behind rocks or trees. Also, "in case of emergencies", you can use the small turbulence piece from the RFR water to make any transition; it looks a bit sharp there, but it's at least believable and looks much better once you've spruced it up with a few rocks and the like (BTW, this also works well with my own waterfall lots):
This is now how the complete river, made with the RFR water, looks like. BTW, I'm using the RFR water for anything far away of the city (mostly in order not to interfere with any lots there), and channel/water lots, like the Neko brooks, at narrow spaces. Diggis water also comes with large pieces that can be used for wide rivers.

1: Note the riverbend here. I'm, of course, sometimes doing just random stuff like that. I'm placing a turbulence piece there, because there will be a rock in the way causing such turbulences.
2: Doesn't look very nice. You sometimes have to live with it. Don't worry, we can cover that with rocks later.
3: On rapids, I'm sometimes splitting the waterway. There are more elaborate examples on that, really, I'm not the best MMP designer.
4: Here, I'm using diggis's water system to make a bridge. Don't worry, we will place a few rocks there later, too. Step 3: Shoreline No riverside is complete with some MMP's at the shoreline. A lot is possible, but for rivers, I'm usually only using rocks and gravel. Sand is usually washed away too fast, and especially in mountaineous areas with considerable amounts of precipitation, the water level changes frequently, thus, river shores with a lot of gravel are very common. (On small brooks or ponds, I'm usually only dragging a thin line of rocks and place grass and cattails right next to them).
So far, I've been using again Pegasus' stuff, and rocks by Murimk and NBVC. You can easily find the latter two by searching for @murimk's content on the PLEX and for @nbvc's content by clicking on the tag I made.
We start with the largest rocks and end with the smallest ones. Murimk's large rocks are IMO very useful for making cliffsides where your terrain/rock mod won't allow that, or on rapids, or in very rocky areas, anyway. You can also see how I'm covering the ugly spot at 2 and how I'm placing rocks at turbulences to spurce up everything a little:
Pegasus' boulders are also very useful, especially because they are circular and thus easy to place; I'm scattering them everywhere, both randomly and where there are some ugly edges in the line of MMP's. I'm also occasionally using Pegasus' gravel and pebble, especially on flat land areas where I'm imagining that the riverbed is very flat and thus, the river is swelling massively in size after a few days of rainfall:
Next on the list are these fabulous rocks by NBVC. Admittedly, this has been the first time I've been using them. I'm just filling everything now with these rocks, leaving a bit free every now and then, and then, I'm filling the rest with the grey stones, by NBVC as well:
After using the rocks, the result could look like this:
And now, I'm filling the rest with the small grey rocks that only take up little space:
Still looks somewhat boring? True. But don't worry: You can barely imagine how much better it will look after turning texture quality to high and adding flora there:
And now, we will add some of Girafe's trees (power search for "girafe" at the LEX, these come with a lot of links). In this case, beech, oak, common spruce, and conifer, to simulate a mixed forest in moderately high altitudes in my country:
And last, but not least, some of Girafe's bushes, as well as Pegasus' leaves from one of his plugins, to cover the floor because grass doesn't grow in forests all that much (I can't remember which one).
Of course, that depends on my mood, which trees I'm using; for grass, I recommend using the green rye grass from the RRP Pasture Flora set:
https://www.sc4devotion.com/csxlex/lex_filedesc.php?lotGET=1886.
You have seen that on this picture:
That's it for now. I'm still experimenting around with how to place MMP's in a both believable and well-looking way, and I've seen much better pictures. Hope you liked it, nevertheless.