Coursing and twisting through the Royal Territory* within Lamara, the Krysa River and its countless tributaries are completely and utterly based. Need fertile land? Honey wake up, the sun has arisen on yet another day in which the soil here in Royal Territory is nice and fertile thanks to the river that provides it with water and minerals. Need easy trade? Babe cut my arm off, we're sacrificing it to show gratitude for the fact that the rivers are all slow moving and easy to move up and down, and that they're usually pretty deep and wide so you can put loads of heavy shit on any trade ships you're using without them getting stuck in the mud below. In fact, take my toes too: we should show gratitude for the bottoms of these rivers mostly having mud instead of rocks, so even if someone does somehow overload their boat then it's not gonna get as badly damaged as it would be otherwise. Need fish? Son, we're going fishing today in this wonderful river with shitloads of fish. Need a defensive advantage in a battle? Boss, you gotta refer back to these rivers being deep, and they have steep banks too. Just natural moats all over the place I'm telling ya. It's especially lovely if some enemies are passing through by boat and we've got all these river-fed trees to cover up our presence so we can attack them by surprise. Wanna make bridges? Grandpa, my teacher said that the strong soil on the banks makes it really easy to build stone bridges over them. Amazing for providing Royal Territory with lots of food and with that high population, amazing for being able to transport resources and people around within it - such as big stone blocks for use in their over-the-top architecture and fortifications, amazing for providing huge advantages to anyone who's on the defensive as any invaders have to either use lots of bridges or travel the rivers by boat and either of those make it easy to trap, surprise and avoid being overwhelmed by numerically superior enemies. *Royal Territory refers to the area of the Empire of Lamara most close-to-directly controlled by the Monarch and where the majority of the population follow the same interpretations of the Lamaran faith and have the same cultural ideals as the royalty are raised to have. It's where the Krysa Kingdom, which conquered the rest of Lamara to form the Empire of Lamara, was located. It's the richest, best developed area of the country and while almost all of it is still controlled by vassals rather than being under direct Crown rule these vassals are generally much more loyal to the Crown than those across the rest of the empire.
The rivers are deep, generally have steep banks, have slow-moving water that's rich in minerals and most banks are home on both sides to forests with rich undergrowths with most exceptions being due to human deforestation and agriculture. The bottoms of these rivers are mostly made of soft mud but the river-banks either have a layer of soft mud over stronger mud or are just made of strong mud. The water is rich in minerals, so the water is pretty healthy once disinfected (slow water -> easy for bacteria to grow numerous) and great for plants. Oxbow lakes (google images it) are pretty common and are generally hotbeds of bacteria and insects.
Lots of fish, riverweeds, some areas having lillypads. The water has a lot of bacteria in areas without many riverweeds but some of the riverweeds excrete anti-bacterial substances. Insects are generally kept from becoming too common by fish eating them but they're there, especially in oxbow lakes where the fish are often wiped out by how easy they are to hunt in the enclosed spaces. River lizards, which have flat and long tails for propulsion in water like crocodiles but lack the bite force that lets crocodiles hunt thirsty land-based animals as well as fish, are pretty common and hunt the fish. River drakes, which are functionally the same as crocodiles but look quite different from them, are still present as apex predators in some areas but for the most part have been actively wiped out by humans due to their human-munching tendencies and aren't the best at growing their numbers because they often kill and eat one another. Birds of prey, stork-like birds and wargulls (seagulls but worse, known as wargulls for how they'll kill one another over the tiniest scraps of food, with it being easy to indirectly kill a dozen or more of them by giving one of them a tiny bit of food when others are watching) hunt any fish that go near the surface, as well as other wildlife in the rivers. Frogs, toads, axolotl-like creatures, other amphibians and even freshwater crabs, lobsters, other crustaceans, otter-like creatures, actual otters (who are in constant conflict with the otter-likes), beavers and all sorts of other creatures occupy the rivers. Aside from that, more or less all the non-flying animals in Royal Territory can swim as otherwise travel without bridges is basically impossible in Royal Territory.
Winter is a thing and generally freezes the rivers, with the ice often being more than three meters thick. There's also a period of constant storms that comes every 15 years or so, in which the rivers often flood. During winter, sleighs pulled by animals become a common form of travel and a lot of boats are even built in a way that makes it easy to convert them into sleighs. The icy surfaces of the rivers are generally flat and smooth, so all that's needed are a snow-plough and maybe some foot gear (like horseshoes, but idk what they're called if you put them on cows) for the animals pulling the sleighs. It doesn't normally snow that much during winter, despite it being very cold, so as more bridges have been built sleighs have become less necessary since regular carts pulled by animals are often just as viable (they're not as usable on the frozen rivers as the wheels often dig into the ice and get stuck in it rather than slide or roll over it).
Wood, fish/meat and fertile land by the river itself. Royal Territory has a lot of iron ore but this is largely unrelated to the river.
Was the center of trade and often warfare before the entirety of the area came to be under the control of the Krysa Kingdom. Once the Krysa Kingdom was made, which had to be done just as much with religious conversion as conquest due to how difficult it is to be on the attacking side in this area, it was just a center of trade... except during civil wars. Attempts to invade Royal Territory through the rivers have been attempted and have generally been unsuccessful. Attempts to invade by land have also been generally unsuccessful. As the fortifications along the rivers have improved, invading the place has only gotten harder. Currently, the mouth of the river is just slathered in fortification. Lamara's capital is on the Krysa River, behind these fortifications (in addition to having many of its own), being positioned where four major tributaries all meet the river at once. The mouth of the river has major towns on either side, each so well-connected to the capital with wide roads and canals between rivers that most don't really consider them to be separate from the capital, especially as it's usually at one of these that the Ark of the Monarch is docked.