Some small bridges

A bridge isn't just a passage it also controls river traffic like a fort and can stop enemy raiders by boat or collect tax. River fees might have so many tolls that land travel and freight can be cheaper. Vast rafts of goods with crafters huts ply some big rivers. Streamlined barges pulled by horses even through tunnels. Bridges can restrict foot traffic and fishing boat rights too. Some squatters might live on pylons or use bridges as shelter. In some places this is unwelcome. 

Bigger bridges are fortified and have toll stations run by local authorities, Sometimes bad guys take over, it's hard to tell really. Trolls and gangs and others might squat a bridge and charge tolls. Knights and barbarians hang out looking for fights.

Bigger bridges have cavities to reduce weight and these have lots of uses.

There are other cruder types and the bigger again giant built bridges of shadelport,

d4 Bridge
1 Single arch style - mostly rural or to allow bigger boats 
2 Dual arch style - for directional river traffic 
3 Ornate style - fancy but also stop enemy boats 

4 Tri-Span Style, possible VIP lane 

type 1 may have one lane or be more narrow
don't tend to have cavities in brings but shore foundations might
type 2+ are larger and may have a d4-1 cavity and two lanes

d12 Rural Bridge Encounter
1 Farmer moving livestock or goods on the wagon
2 Merchant wagon with escort
3 Hermit lives under bridge d4 1=mad 2=holy 3=drunk 4=runaway youth
4 Animals using the bridge on own d4 1=goats 2=deer 3=sheep 4=auroch cattle
5 Bandit gang or highway robber charges a toll
6 Hostile raiders have taken bridges to block and rob traffic
7 Warrior looking for a peer to challenge d4 1=knight 2=barbarian 3=monk 4=wizard
8 Tax collectors or official toll collectors with soldiers
9 Holy travellers d4 1=high priest with servants and guards 2=lone country friar 3=monks or nuns with escort and holy books or relic 4=inquisitor hunting witches, ghosts, cults, undead or lycanthropes  
10 Itinerant workers camping and drinking
11 Monster has occupied bridge to charge toll d8 1=troll 2=ogre 3=giant 4=wyvern 5=manticore 6=harpies 7=sphyx 8=dragon
12 Supernatural being d4 1=deceased spirit in disguise 2=nature spirit 3=demon demands sacrifice to pass 4=angel brings a message for mortals you will do

You might want to choose a bridge-type based on the encounter if you are world-building as you go.

d12 Hidden Bridge Cavities
1 Storage place sealed for wartime emergency by the crown
2 Large space occupied by d4 1=homeless 2=children 3=vermin 4=gang
3 Entombed remains d4 1=criminals sealed inside alive 2=sealed tomb 3=undead haunted crypt 4=prison for vampire or mummy weakened by flowing water
4 Sweatshop with miserable workers locked in under contract or slaves
5 Workshop and storage for road and bridge works
6 Secret cult meeting place with hidden entrance nearby
7 Lodge or meeting hall of local society or club or guild
8  Animal lair d4 1=giant frogs 2=giant otter 3=giant river rats 4=giant leeches
9 Abandoned secret chamber d4 1=treasure 2=oubliette 3=shrine 4=cavern entrance
10 Secret home d4 1=witch 2=druid 3=mutant 4=thief
11 Monster lair d4 1=kelpie 2=algoid 3=kappa 4=hag
12 Supernatural being d4 1=spirit folk 2=faerie 3=imp 4=hainted spirit with remains sealed inside