Seta River mouth
Many rivers flow into Lake Biwa, but there is only one river that flows out of the lake - Seta River. It exits the lake at Ishiyama, and the bicycle path that stretches along Otsu Waterfront extends along the riverside as far as Ishiyama Temple. You really should visit this fantastic temple, but it's worth a half day at least. If you want to cycle to Uji, you probably shouldn't visit Ishiyama Temple on the same day. The river is very wide between the lake and huge Araizeki weir gate, and you will likely see rowers practicing their craft because there is a rowing club at Ishiyama.
Beyond Araizeki Weir Gate, the river narrows, and the current hurries toward the rapids at Kaeru-Iwa (Frog Rock). You could stop and go white water rafting here... but perhaps not if you want to reach Uji. However if you feel like a break from riding, and have the energy to climb a lot of steps, Tachiki-Kannon Temple stands on the hillside opposite the river. If you don't have that much energy, you could just sit on the river bank and try to figure out which one of the fantastically shaped rocks looks like a frog.
Ujitawara and the bridge to no-where
If you turned off and crossed the bridge just after Kaeru-Iwa, the road would eventually take you to Shigaraki, but in this story our destination is Uji, so I'll write about that road another day. Continue to ride along the river and enjoy the wonderful scenery. You can catch glimpses of an unused railway line on the far bank, unless the forest has swallowed it completely. You will reach another bridge with a sign post to Ujitawara, where they grow a lot of delicious Uji tea. Cross the bridge, but continue along the river. I think this is where the Seta river becomes Uji River. Somewhere along here, there is a suspension bridge. In summer it is overgrown with vines. On the other side there is nothing! Perhaps there was once a road, and it was washed away.
Amagase Dam to Uji Bridge
A little further and you will reach Phoenix Lake and Amagase Dam. There is another bridge just after the dam, and now you need to make a tough decision. Which side of the river will you cycle down to Uji Bridge? I can't help you, because both sides are beautiful!
A round trip from Kyoto
Be careful of the traffic - the road is not wide, and there are no bicycle lanes. Except for the stretch of road from Uji Bridge up to Amagase Dam the grade is never steep, so this road is great whether you tackle it from the Uji end or the Biwako end. When I lived in Yamashina, I usually preferred to go from the Uji side. I would cycle the path along Yamashina River to Rokujizo, endure the unpleasantly busy road to Uji Bridge, lunch at Amagase Dam, and proceed in a leisurely fashion to Lake Biwa in time to enjoy the evening light over the water. I would then cycle over the hill back to Kyoto. The round trip was a little over 50 kilometers.
Best time of year?
What is the best time of year for this ride? I have ridden it from early spring to late autumn, and it was different in each season, but always wonderful. Probably I would say summer and autumn were best.