I was just wonderding if the technology seen in earlier civilizations was temporarily lost, or if they were not common due to other focuses, decentralized feudalism, etc.
No, it wasn't. I know about this period in relation to Umayyad al-Andalus (islamic spain, c. 8th century-11th century) so that's what I'll focus on. Aqueducts and other forms of plumbing & irrigation were a common feature in al-Andalus. Under the reign of the Emir Abdul Rahman I (756-788 or so), a palace called the al-Munyat al-Rusafa was built. It was the first palace built entirely newly in Cordoba by the Andalusis, approximately 3km northwest of Cordoba. it's a ruin today as it was burnt during the first fitna of andalus (a post for another thread - basically a giant civil war/collapse in the early 11th century), but we know quite a bit about it thanks to archaeology and period writers. For instance, extensive gardens and unusual plants, flowers & fruits were a central and distinguishing feature of Andalusi palaces, especially during the Umayyad/Cordoban era. And they were, as Ruggles notes Ibn Said's description, dependent on plumbing - especially aqueducts, which we know they had both from archaeology of extant ruins today as well as writers like Ibn Said who described them in detail.
Madinat al-Zahra is another great example, of this, especially because it was a city built from scratch around a new royal palace during the reign of the Caliph Abdul Rahman III. Madinat al-Zahra included extensive aqueducts that crossed uneven mountain terrain and utilised bridges and tunnels along the aqueduct structure to do so. The aqueduct thereby deposited water into a basin on the north side of the city, whereby it would then pass from the basin into a closed, vaulted resevoir from which it ran through the rest of the palace and the city, to the plains below, and even included channels, pipes, and underground sewers. We know from archaeological excavation that the aqueduct portion itself utilised recycled parts of old Roman ruins and in other parts was built over the top of them, but the overall plumbing system was newly designed and built by the Andalusis.
They were also extensive builders of fountains, pools, and wells. Ruggles also points out that the plumbing features of al-Andalus were so extensive & and detailed that contemporaneous people were very proud of them, and so Andalusi writers often painstakingly described the various aqueducts, terraces, fountains, channels, and norias (waterwheels that move water into aqueducts) in so much detail that the plumbing & hydraulic world of cities like Cordoba is easier to reconstruct & map out than some of its structures & palaces. Worth noting as well is that although the ancient Romans did rule in Iberia at one time, knowledge of aqueduct construction, agriculture and hydraulics wasn't transmitted to the Andalusis domestically from Iberia but from Syria. In fact, if we go back to the al-Munyat al-Rusafa in Cordoba, significant parts of its designs were influenced by another palace in Syria. The people in Syria and indeed, the pre-Iberian Umayyads inherited a lot of this from the ancient Romans. So the knowledge of the hydraulic technology you're referring to certainly wasn't lost, not even temporarily so, and this technology was in fact common, at least in al-Andalus.
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