This comes from a few discussions I've had on Discord. I find the river hex penalties to be confounding. I've generalized several times that "Rivers = death." 

So at present all units moving into a River Hex stop immediately. Any unit that has moved into a river hex will be attempting to cross the river and should receive the penalty for a turn. If a friendly melee unit that has ended it's movement in a River Hex is engaged with by an enemy unit they will not finish crossing the river hex and should receive the penalty on the next round. This situation is perpuated anytime a friendly melee unit is in combat on subsequent turns. However a friendly ranged unit engaged on a river hex only receives the strength penalty on the first round, they do not need to cross the hex to engage enemy units. 

A ranged unit that has attacked a melee unit that is attempting to cross the river does not have to attempt to the cross the river and should not receive the penalty.

A (melee) infantry unit that has attacked a melee unit that is attempting to cross the river also has to attempt to cross (or may have to enter the river for combat) and should receive the penalty.

A (melee) cavalry unit that has attacked any unit that is attempting to cross the river also has to attempt to cross (or may have to enter the river for combat) and should receive the penalty. 

If a (melee) unit engaging a unit from the rear hexes does not cross or end it's turn on a hex with a river it ignores the river in the hex attacked.

If a friendly unit has not been engaged with on the round it entered the space with a hex then on the next turn the strength penalty goes away - or, at most, lasts one round. In this regard you might adjust the penalties a bit. -2 engaged crossing the river. -1 crossed a river on the previous round. At this point movement along the river becomes a bit more complicated. I don't know if you'd be able to correctly program/predict which side of a river a unit is on reflective to enemy units. But if a unit continues to move along a river it might be appropriate and simpler to keep the -1 penalty, as it's strategic options are limited in scope by the river.

I would estimate that bridges should effective remove the river from consideration. In fact a unit defending on bridge should grant different penalties. -1 str if it ends on a hex with a bridge (restricted movement) to attack another unit, but if it was on a hex with a bridge and gets attacked the attacking units get a -1 str penalty (restricted movement), even if it's a ranged unit. Although I just had another thought. Depending on how much "road" technology there is, perhaps bridges allow units to ignore rivers in later eras or with specific infrastructure technology as the earliest bridge might not be more than a small makeshift bridge or in some case maybe even a rope bridge.

Shoot. Okay. I think I have a simplification curtosy in part to Waykot from discord.

Moving into a hex with a river gives you a riverside indicator relevant to the side of the hex you crossed to enter the hex. Attacks from the opposite 3 sides of the hex by (enemy) melee units trigger combat on the "uneven ground" of the river bed for both units: everybody gets the strength penalty. Attacks from the opposite 3 sides of the hex by (enemy) ranged units triggers combat on the "uneven ground" for the melee units: the defending unit gets the strength penalty. Any attacks from the same 3 sides of the riverside indicator happen without penalty. On the turn the unit moves out of a hex with a river, it gets the penalty for crossing the river, and you can tool tip it that way but it's all the same "uneven ground" bonus. I can make graphics if this isn't readily understandable.

Note that some cavalry may receive a double penalty when fighting on "uneven ground". Just seems likely ;)

PotatoesAreBland wrote:

Bridges should still factor in, removing the river bed combat. I guess it'd fit as an infrastructure technology.

Masonry creates bridges on any roads that cross rivers between your cities/city-centers. Or. Maybe it could be a cheap improvement that creates bridges over river hexes in that territory which also grants +X fortification to the space.

i would think crossing a bridge could potentially provide a type penality too, if engaged in combat.  As it's a funnel point and and severely restricts what maneuvers your units could do, apart from charge forward, retreat, or hunker against a wall (if any).

So I'm really liking the idea of friendly bridges providing a small fortification bonus for friendly units - a bonus which can be expanded upon by further technologies or joined to other fortification buildings - outposts / catles / etc. They could also function as a preferred reinformcement position. Rivers become much more defendable at certain points. Also - capturing forts/castles might end up being more of a grant strategy decision. In endless legend the towers were a little bit random, this would help direct strategy decisions in both city placement and army movements.