Minecraft – How to keep the same colors during daylight cycle

What you should do is make the water only one block deep, so that it all levels out. See the question that ZoogieZork posted. After that, you can dig as deep as you want and the currents shouldn't appear.

In general, here is a good technique for building rivers (top-down view):

█▒█▒█▒█▒█▒█▒█▒█... | █ = block
.█▒█▒█▒█▒█▒█▒█▒█.. | ▒ = water source
..█▒█▒█▒█▒█▒█▒█▒█. | . = empty block

First, you want to dig out a path for your river. Place alternating blocks and water sources along where your river is dug out so it looks like a checkerboard. Then, destroy all the blocks. Because there are water sources every other block, the space created by the blocks being destroyed will turn into source blocks themselves and create a nice and smooth river.

In your case, you would probably be able to build this pattern right up to one block before the lake. When you've destroyed all the blocks and the river is nice and smooth, you should be able to connect the lake and the river and the connection between them should level out.

I solved the problem temporarily by switching one farm to regular bricks. This is still apparently a bug, I'll go file a bug report for Sengir to see, there's no reason why multiple farms should not be able to use the same farmland, as this drastically limits the number of functional farms you could have. If anyone else has the same issue, try changing one of the multifarm farmlands to a block type not used in other identical farms until Sengir releases a patch.

EDIT: This only temporarily fixed the problem. Within 20 minutes it again didn't recognize the farmland, despite the bricks. Will have to figure out another solution, as I can't be running back to the farm to change out the blocks every 20 mins.