why is it texas reservoirs warm up on the north side of the lake first in spring and in idaho at least my new home reservoir is the opposite?

Have you had strong winds out of the north? The wind can blow the warmer surface water across the lake and make a couple degree differential.

Is the south side shallow and dirtier? Dirty water and shallow water with dark bottoms will absorb sunlight faster so clear, deep water will normally warm slower.

Are the dams on opposite sides of the reservoirs? The dam side is the deep side and all that extra water acts like a heat sink. The ratio between energy added and temperature gained is fixed(4.184j/kg/degree c). But, surface area plays a large part in the rate at which energy can be gained or lost. So more surface area relative to volume means faster to heat and cool. Finally, I'm guessing the reservoir in Texas was an earthen embankment with a relatively long and low dam face. The Idaho dam on the other hand is probably concrete arch or concrete gravity plug. It likely has a much shorter and taller dam face.

So, my guess is the dams are on different sides of the water bodies and the water bodies are classic upland vs lowland reservoirs. Texas's big wide flat water body can gain and lose heat faster than the narrow deep water body in Idaho.

Dams are both on south side. I dont really agree shallow vs deeper when talking fishing reservoirs. The trolling motor temp sensor is only 2 feet deep and were talking early spring. Only the top of the water column is heating up whether its shallow or deep. Must be the snow melt runoff up in canada for the north end is colder than the lack of snow in idaho for the south end

Elevation differences & longitude differences combined with sun exposure might be the answer. Most of the Idaho reservoirs are river fed while Texas is not?

6 hours ago, blanked said:

Only the top of the water column is heating up whether its shallow or deep.

This isn't necessarily true. In clear water where light is penetrating more than the 2' trolling motor depth, the sun is warming the bottom. So if you're in 4' of water in a lake with 6' visibility then the sun is warming the lake bed and in turn the water along the bottom. In order for water to heat up from light (radiation technically), there needs to be something in the water to absorb the energy. Clear, deep water doesn't absorb the light very well. Particular matter does whether it is silt, algae, tannins, or anything else suspended in the water.

Also, you have mixing. If there is wind (or other current) then even the surface warming that is happening is getting moved or mixed into the rest of the lake. So while it's technically true that the surface was warmed, that warmth was moved away from the surface.

6 hours ago, blanked said:

Must be the snow melt runoff up in canada for the north end is colder than the lack of snow in idaho for the south end

This is very possible. The easiest way to warm up a lake? Pump a whole bunch of warm water into it. I have one nearby (big, cold, clear, deep) that has water pumped into it to fill it. The main lake was 42 degrees on Friday but where they are pumping it in was 47. My dad's home lake is fed by a wide, shallow river in the mountains and when the rain or air temp in the spring is cold like it is now, the upper part of the lake is colder than the dam area. Neither of these have to do with compass directions though, it's just the flow of water into them.