There is generally water sub-surface in most river beds. This will keep eggs alive even during prolonged dry spells, especially eggs that have been buried by channel changes from previously high flows. Upon emerging sac fry will often migrate well into this hyphoreic layer to await button up, and will sometimes migrate great distances underground, even coming up in ponds behind dikes, one of the reasons gravel mining in the flood plain is so dangerous. Salmon fry have even shown up in tap water drawn out of shallow wells drilled to close to the river The water you see in the river is only a fraction of what is there, most of it is underground. This underground river is flowing, and it readily mixes with moderate-temperatured ground water, so freezing is unlikely. However, a prolonged drought like we have had has likely had an effect on the water table, especially in watersheds with a dense population and lots of wells that have unnaturally drawn it down over the summer. The hyphoreic zone drops with the water table, and if this keeps up those eggs may not make it. If you have a well, don't use much water.