While our beloved trout are winter spawners with a long in-gravel incubation period that releases babies into the river when things start to warm up and hopefully become easier for little uns, most other river fish species egg-lay in the spring, with much shorter incubation times.
Right now, early April, grayling are spawning in my local river (the Itchen) and bullheads have very recently done so too. This video, courtesy of WTT member Martin Smith, shows brook lampreys doing their communal spawning thing, on the Thornton Beck in N Yorks. The adult lampreys use their weak, sucker mouths to move stones, creating a shallow depression into which their eggs can be laid. The resulting blind juveniles, called in the lamprey species ammocoetes, live for perhaps several years in silt beds in the river, filter-feeding on organic material.
They change into adults in the autumn, developing eyes, a silvery coloration and distinct fins, before spawning, then dying, the following spring. These animals, not technically true fish, are incredibly interesting and very well worth reading more about. Find on the internet the brilliant paper by Peter Maitland for Natural England: Ecology of the River, Brook and Sea Lamprey.