Creating Realistic Terrain
Creating realistic terrain involves more than simply sculpting out a feature and plonking it on the landscape. To truly create a believable landscape, the entire process of natural river creation has to be understood, so that rivers can be created that flow naturally into the landscape.
The Water Cycle
Water does not sit still. If a world has weather, it has moving water, continually circulating. This circulation is what keeps the rivers you desire, full of water, and flowing across the land. Water evaporates from any standing body, and flows through the air into greater and greater clouds, rising and rising until it meets colder air, at which point it rapidly condenses and falls as rain. If the clouds rise too high, the water freezes and snow forms. The height of the land forces clouds higher, which is why it snows over mountains.
This also means that rivers are controlled by wind direction. If the prevailing wind is blowing east, then water collected from bodies of water to the west will evaporate into clouds, and rise over land heading east. Some will fall back as rain, some will survive to reach the mountains, and most will deposit on the western slopes of those mountains. The wind will flow over the mountains, but the last of the moisture will have frozen and fallen as snow or sleet. It will hardly ever rain on the eastern slopes, and any rivers that form, will flow west- where all the water has gathered.
If you follow a river right the way down from it's start point in hilly or mountainous ground, flowing with gravity always downhill, you will find any river is fairly easy to split into two or three stages, which in geography, are formally titled reaches.
In the upper reach, we have a steep river, as the initial channels form in very uneven ground, or, in the case of underground rivers, in lots of tiny tunnels through the rock. Water speed is usually swift, and lots of erosion occurs.
In the middle reach, or river valley, the river broadens out, and as it does, the water tends to flow slower, carving a more gentle, V shaped valley out of the surrounding rock.
Finally in the lower reach we have the delta, or flood plain. The flat lands where the river empties into the sea.
Each reach has very different geography, and to properly recreate a river we require all three stages, carefully blended into each other. We will begin by looking at the upper reach, or steep river.
The Steep River
The source of a river is always on ground above sea level. Rivers flow according to gravity, and gravity flows downhill. Most rivers start in mountains as the excessive snow, and sometimes glacier output is an excellent source of water. Failing that, a river can start from a spring - a place where a higher, porous rock meets a lower laying non-porous rock strata, and the groundwater flowing from higher ground, bursts forth and flows above ground. This is how rivers start in hilly terrain, and they always start partway down a slope, never at the top, but always where two differing rock strata meet.
The steep river is not one single river, but many tributaries, erach flowing a separate path until they slowly join up, one aftyer another, the river getting wider each time. Tributaries can join up separately to form small rivers, which themselves merge with other small rivers further on, to create one large river. Each tributary flows fast enough to pick up loose stones, and dirt. Over time, the effect of the rocks banging on the river floor, and the water lifting particles out, carves a channel out of the rock.
These channels are always V shaped, because the water always flows fastest in the middle. Over time it wears down into the rock, until the sides collapse in, and the river carries away the loose material - a V shaped depression forms with the river in the middle.
The River Valley
The river changes dramatically once it hits gentler slopes. Because gravity is less intense, the river slows down a great deal. More tributaries join in, and the river rapidly widens. It still cuts a path downhill, and it still makes a channel in the same way it did in the fast river.
However, as the river is flowing more slowly here, it will begin to bend, and flow in loops. Maybe initially one side of the river had a softer rock that eroded more quickly than the other. This then put more pressure on the opposite bank further down as the water that had eroded the softer rock, then cut sideways across the rest of the river. Whatever the reason, once an oscillation starts, it magnifies over time.
The water on the inside of the bend flows more slowly than the water on the outside, and slowly deposits stones and silt it had carried in suspension, on the bend itself, building up a gradually sloping river bank on the inside of the bend.
On the outside of the bend, the river is flowing at its fastest, and like with the fast river earlier on, is actively ripping small stones and silt out of the bank on the outside of the bend, pulling the bend very, very slowly, further round. The outside bank is usually a sheer drop, complementing the inside bank's gentle slope.
The Flat Lands
The point the river flows through flat or nearly flat land, near the sea, is called the flat lands, or flood plain. It still oscillates, and still carries very fine, ground down sand and dirt with it, known as silt. This is extremely good soil, and all the stony bits have been worked out, as they were too heavy for this stage of the river to carry. Because the river flows so slowly now, almost unpropelled by gravity, there are no actively eroding banks, and the river is bordered by gently sloping levees, made up of deposited silt. This close to the sea, rain is common, most of the moisture of the water cycle is still in the air, and after particularly heavy rains, the river will sometimes burst its banks, flowing out over the flat land in a flood. This flood deposits silt over a wide area, creating massive areas of very fertile farmland.
Silt is also deposited on the bed of the river, which slowly makes the river shallower. The silt levees are built up at the same rate, and you can eventually finish up with the situation where the river is actually flowing above the level of the flood plain around it.
The River Mouth
Finally, the river meets the sea. Depending on the power of the river, it can either form an estuary or a delta.
There you have it, the stages your river will have to have, to truly be a believable flow of water from the high ground to the low ground. Only once the river itself is in place, can you begin altering and adapting to add traces of civilisation etc. Civilisation adapts the landscape, not the other way round.