The Douro River is wide and deep and flows in a great valley. For a tour-boat passenger, the scenery is famously wonderful, but the structures that are needed to control the river and allow land traffic between its banks are fascinating. Five great dams and their associated locks were used to tame the flow and maintain navigability. Seemingly countless bridges maintain surface traffic between the banks. And river-side structures, some impressive, some mere ruins, are endlessly intriguing. 

The locks are impressive. The highest has a lift of 115 feet. The machinery and scale entertain the passengers.

A rental cruiser heading upriver exits a lock. 

The process, repeated lifts heading up river and repeated drops heading down river, make for dramatic viewing. 

Passengers on the bow have a good view as the locks' doors open.

This passenger wore sparkly deck shoes while watching the doors open from a seat in the bow of the Amalia Rodrigues. 

The ships are built to the maximum allowed through the locks; passengers check on clearances.

The bridges are equally impressive.

These bridges cross the Douro in Porto.

These, farther upriver, connect the banks.

Tributaries need bridges, too.

Clearance beneath bridges can be minimal. The ship's superstructure (here, the canopy over the viewing deck) can be lowered.

A passenger checks clearance at close hand.

Entertaining fact: river height varies with release from upstream dams and storms and winter runoff. There is no guarantee the ships will clear under all conditions.

Riverside structures run the gamut.