This is RPG-ology #18: Waterways, for May 2019.
We mentioned rivers, lakes, and other bodies of water in connection with maps when we talked about Country Roads and again when we were talking about the placement of Cities, but we barely touched on them, more from the perspective of their influence on other aspects of our maps and our worlds. Maps and worlds, though, are complicated things, in which everything influences everything, and understanding how waterways work will help us put together better maps.
This is difficult for me, because so much of it seems obvious to me so I expect it will be obvious to everyone else. However, I have the advantage, as I think I mentioned in Shock, of over a thousand miles of long-haul canoeing, so I am perhaps intimately familiar with rivers and lakes and ponds and how they work. I thus hope that I’m not telling you too many obvious points, and that some of this proves to be practical. Let’s start with some terms.
A river is pretty much any waterway that flows downhill. They can be big or small, swift or lazy, shallow or deep, straight or meandering, rocky or clear, in any combination. Smaller rivers are often called brooks, streams, creeks, and similar diminutive titles, but the only significant difference is the attitude of the people toward the waterway and the probability of it going dry, which rivers rarely do.
Lakes and ponds are usually found as interruptions in rivers, and they are distinct from rivers in a significant way. A lake or pond is formed in essence when water pours into a natural basin and has to rise to the level of an exit point. Because of this, the surface of a lake or pond is level, while that of a river is always sloped–if you look at the accompanying photo, you can see that the downstream end is downhill. In the vernacular, lakes and ponds are generally distinguished by their size, but technically they are distinguished by their depth: a pond is shallow enough that water plants such as waterlilly pads can root on the bottom and grow on the surface, while a lake has at least some areas in which it is too deep for that. Lakes and ponds are sometimes created intentionally by the use of dams, built by people or sometimes by animals, most typically beavers.
It is difficult to distinguish a sea from a lake in many cases. Seas tend to be the terminus of rivers, at least one and often several, but most of them either drain into or are contiguous with the oceans, which are also sometimes called seas but which as a word tends to refer to the vast expanses of water separating the continents. The two exceptions to the drainage rule are the Dead Sea, which is constantly evaporating and so is too salty to support marine life, and the Mediterranean, which also loses its water to evaporation but is large enough that its salinity, although elevated, is not inimical to such life, and fishing and the like are active there. (It is easy looking at a map to suppose that the Mediterranean drains into the Atlantic Ocean through the Strait of Gibralter, but the water there is mostly flowing very rapidly in the opposite direction, salt water from the Atlantic constantly replenishing the losses in the Mediterranean through what some have called the world’s largest waterfall. There is an undercurrent flowing westward as a small amount of dense saltier water goes from the Mediterranean to the Atlantic, but the bulk of the volume is inflow from the Atlantic.) It is also the case that seas tend to be salty while lakes tend to be fresh, but this is not a hard and fast rule, there being a number of salt lakes in the world. Part of this is because the distinctions between lakes and seas are not made in all Indo-European languages, and English has often translated words strictly that were used loosely.
A passageway that connects two bodies of water of the same or similar level is usually called a strait (or sometimes straits), usually unlike a river because water flows in and out both ends generally with the shifting tides. If it is wider or longer, it is often called a channel, but this word also refers to the best path through a river—rivers tend to carve a deeper groove through which most of the water travels, and boats and ships navigate through these deeper sections either with or against the current. In modern times, these channels are marked by buoys, red buoys to the right when traveling upstream (“Red Right Returning”), black buoys marking the other edge.
A bay frequently appears as a brackish (that is, salty but less salty than the ocean) body of water connecting a river to a sea or ocean. As the tide rises, water from the ocean pours into the bay, often forcing its way upstream reversing the “normal” flow of the river; the Delaware River is brackish as far upstream as Trenton, New Jersey, about eighty miles upstream, about fifty feet above sea level, and this reverse flow is often used by ships to navigate to inland ports upriver. As the tide ebbs the bay drains into the ocean, and the river into the bay, and fresh water makes its way downstream to wash away the salt. Because of the backwash, those upstream ports have rising and ebbing tides, but these are out of phase with the coastal tides that drive them, often by as much as six or eight hours, depending on how far upriver you go. A very small bay-like inlet is usually called a cove; a lagoon generally is a type of coastal pool that fills from ocean spill when the tide rises over its banks, and then slowly evaporates, frequently not completely before being refilled.
A wadi is something like a river, but significantly different. Common to inland tropics such as Africa, the wadi is a watercourse that floods and dries in a seasonal cycle. During the “rainy season” water falls in the highlands and flows down very like rivers, working downstream and gradually covering thousands of acres of ground, pooling but flowing, spreading over wide expanses of open space. Animals are aware of the seasonal cues, and migrate toward the anticipated flood; plants desiccated from drought spring to life and blossom. For a few months it is a lush wet marshy world, water plentiful, wildlife active. Then gradually it all evaporates, leaving the dry grassland to wither in the heat, as the animals scatter to places better able to support them during the drought. The water from a wadi never reaches the seas, soaking into the earth and evaporating into the air long before joining any other watercourse. Wadis do not support ship traffic or permanent settlements, because the water level is non-existent for a significant part of the year and rarely deep enough for more than the smallest craft.
Swamps, marshes, bayous, and deltas all tend to be areas where a river spreads out to a shallow wide area, usually with a channel passing through it somewhere but often a confusing labyrinth of waterways leading to dead ends and shallow muck. Wadis do support marshes and swamps during their wet periods; bayous and deltas tend to be at points where the river meets the sea, and are brackish like bays.
Now, this might sound obvious, but water falls from the sky. Really all of it does. Water in wells and water coming from springs is water that fell from the sky and soaked into the ground, then collected atop or between layers of rock and either sat waiting to be collected or built up pressure from gravity until it spurted through an exit. It gets into the sky by evaporation, the vast majority of this from the vast expanses of tropical oceans—if your world does not have vast tropical oceans, you will have a lot less rain, and a lot less fresh water. Evaporated water, water vapor, is held in greater quantity in denser warmer air; if the air cools or becomes depressurized, it cannot hold as much water and so releases it. This is why so much precipitation (rain and snow) falls on mountains: warm moist dense air currents are shifted upward into cooler low pressure altitudes, and can no longer hold as much water. From there it collects in streams or soaks into the ground.
There is an interesting atmospheric phenomenon at this point. As water falls, it washes carbons out of the air, turning into mild carbolic acid. It has always done this; this is not a modern result of air pollution, although air pollution does contribute to it. Carbolic acid which lands on dirt and soaks into it decays and releases its carbons back into the atmosphere. That which lands on rock and flows into streams dissolves the rock, creating calcium carbonate which washes downstream into the oceans, burying the carbon for millenia. That’s not really useful to this discussion, though, so ignore it.
Technically, a well is a hole dug deep enough to hit what is called the water table, the level under the ground where bedrock prevents water from seeping deeper, and so has water refilling it constantly from the surrounding lands. It is sometimes confused with a cistern, which is a dry hole usually lined with stone designed to catch rain when there is rainfall and keep it deep and cool in the ground during the dry seasons. The famed Jacob’s Well is actually a cistern.
This is also obvious: water flows downhill. Because of this it is constantly “seeking” the lowest point, and that means it collects into fewer larger rivers. If it pours into a low point—call it a basin—it collects there, rising as a pond or lake until it rises over the lowest edge. A lake can have several rivers feeding it, or no rivers feeding it if it is fed by a spring or springs below its water line, but rarely does it have more than one draining it—odds are good that there will be one lowest point, and once the water starts pouring through it erosion will make that point lower. If the lake is filling faster than it is emptying, it might rise high enough to begin spilling from another point. However, most typically these are near enough each other that the streams soon join creating an island at the head of the river. If the two streams are headed in different directions, it is most commonly the case that one of the outlets will erode until all the water passes through it, the other becoming dry unless it is fed by other water sources below.
Where the ground is steeper, the water moves faster and generally straighter. It follows the lowest ground, but in doing so carves the path deeper, sometimes wider, removing the dirt and softest stone. If it comes into a pocket of harder stone, it will be turned, but the turning will create swirls and eddies which often drill deep spots in the riverbed. The northern reaches of the Delaware River are frequently shallow enough to wade through, but where it turns sharply at Narrowsburg, New York, there are whirlpools during flooding and the depth at the curve is over a hundred feet deep. Rocky rapids form where the ground is too hard to erode easily and the slope is steeper, as the river becomes forced into a narrow space often between high banks and spreads over the area to become swift and shallow, the irregular bottom redirecting the current in directions difficult to predict without surveying in advance.
Where the ground is less sloped, the water spreads to cover a wide path and flows more slowly, but still tends to follow the lowest ground and carve a channel. In older sections of the river these channels are often meandering, and as the water ultimately settles into them they form snake-like slow rivers with very little noticeable current, frequently surrounded by marshy ground, meadows, and flood plains.
As rivers join, they become wider and deeper, and usually become straighter as the land is less able to resist the flow of the water. These wider deeper rivers which ultimately reach the sea are frequently navigable by ocean-going vessels, and as we noted are also subject to reverse flow when the tides rise, thus brackish but also easier to navigate upstream. They will carve deep sections particularly at curves and bends in the river, as in Narrowsburg, creating good ports at such curves, considerably more so than along straight paths. Upstream of a certain point ocean vessels, which have deep drafts to provide stability in rough seas, give way to shallow-draft river boats, able to navigate farther upstream.
The same currents that form harbors on rivers do so where rivers hit the seas, which makes such points doubly convenient for trade, as a port there accesses both the oceans and the river. Such harbors are also created where the coastline recedes sharply, as ocean currents form eddies which create depths near the shoreline, although if the surrounding ground is low there will probably be a river outlet there, and if not the deep water is likely to be surrounded by cliffs, making for good anchorage but a bad place for a port.
So to summarize, most rivers begin from streams in mountains, flowing downhill and collecting into larger rivers, forming lakes in low spots, rapids over steep rocky ground, meandering courses over flatter softer ground, ultimately becoming large enough to support riverboat trade and then ocean vessels, subject at the downstream end to tidal backflow, emptying into seas and oceans sometimes through intervening bays. Harbors form where currents have carved significant depths, usually at the mouths of the rivers and at river bends.
Now you have some idea of how to put the waterways on your maps.
I have omitted canals from this discussion. Men build canals usually where there are two disconnected waterway systems near enough to each other that it would be commercially profitable to be able to run boats or ships between them. Usually these involve mechanical locks which enable the control of the flow of water between the two systems, particularly if they are not at the same level, and typically because such canals often have to cross ground that is higher than either of the waterways (the reason the waterways haven’t flowed into each other). Sometimes canals are built to get around sections of a river that are not navigable, if there are navigable sections upstream of falls or rapids. They are a lot of work to build, operate, and maintain, and if neglected gradually deteriorate.