It is indeed as seasonal as others have confirmed in reply to your question and as numerous photos illustrate. I knew about this and was excited when I passed through Portomarin a few years ago towards the end of October and discovered that I could actually walk onto the old bridge and stand in the middle of it - an eery feeling because at the time the water level was more or less equal to the walkway on the old bridge which has no guardrails ... you feel as if you are walking and standing on the surface of this vast water in this case ...
Your comment made me curious and I found the Belesar reservoir website with real-time data as well as data for the years 2021, 2022 and 2023 and average data for the last 10 years:
El embalse de Belesar acumula 530 Hm3 (80.92%) -14 Hm3 MENOS que la semana anterior.
www.embalses.net
You write that you passed Portomarin a couple of weeks ago, i.e. in September 2023. I guess this means that you saw the river/reservoir area near Portomarin at a very low water level. At the beginning of August 2023, the mayor of Portomarin
stated that at this point of the summer the level of the reservoir is quite low, so that you can already see the houses of the old Portomarín and the old bridge which had been used for crossing the river Miño until some 60 years ago. However, he also pointed out that the water level in August 2023 was
not yet as low as last year when something unheard of ("cosa sin nombre") occurred, i.e. the water level was lower than ever before during the past six decades.
I guess we could take bets whether it will take another 60 years before the water level of the reservoir/river near Portomarín gets as low as in 2022 and 2021 or whether this becomes a more frequent occurrence.