What is the distance from the river to the tank.
How deep the ground freezes.
Does the river freezes ?.

What is the height difference between tank and river ?.
Where is the tank located and winter protected ?.

What are going to use for water pumping ?.

I don't think I saw that Moose, geuss I missed one.

 I think you could do pretty good with a cistern out there. I've been around alot of them in the warmer states down here, for the most part people have not had much problem. Grand parents and some older friends had them most of their lifes in Ohio, all lived nice and healthy to a good old age. Men around 70 or so, women even older, I think Great grandma was about 100, grandma was in her 90's I think.
 I used a make shift one for a couple years before I had a well put in here, we never got sick, though we did not drink or cook with that water, we filled other jugs for consumable water.

I dought you would have any health problems unless it was because of the out of use time, the ones I know of were in constant daily use year round. Also filled with rain water or well water, not river. But I bet your river is better water than many wells anyway and also better than rain water in many areas too.

I would insulate it good to prevent freezing as much as posable. Stock tank heaters are cheap (here) and if you have power for it I would put one of those in the cistern also.
 Maybe use the solar pannels you built or build a genny for out there, a few batteries and you could power a stock tank heater enough to prevent freezing.
 When you were at Harbor Freight did you see the stainless steel 12V water pump? I think they are about $50, would probably be good for the cistern pump and not have to have a genny running, but you would still need the larger pump for the lift from the river to the cistern.

Something that should work good for you and cheap too, I did this before I had a well here and I was hauling in water from other places in cleaned out junk hot water tanks in the back of a 1 ton box van. Several years! Your river water is better than some well water I hauled!

Buy yourself a swimming pool for at the river :)
No joke, I used one of those 2' or 3' high ones with plastic sides and rubber bottom, not sure how big around. You can get alot of different sizes so get one that holds the volume of water you want to store. Back in those days we had alot of winter days at 20f and some down to -5f, when those pools froze they did not break. Although with -40 they may get brittle, but they should be ok since you won't be messing with them if they get that cold. Warm water will keep the pool warm, and it will probably be empty if your letting it get that cold.

I think I paid around $10 then. Mine just sat out in the yard, I covered the top with black plastic to keep it clean and keep light out, duct taped it around the sides. Stuff likes to grow in light better than the dark. You would probably want to build an insulated box around it up there and maybe use the tank heater sometimes also. Being the fexible type you fold or roll up and open top, when empty it would be easy to clean out, wipe it out with a bleach rag and rinse well. I would want to do that after it sat empty for a long period if it had been warm. Kills any germs and bacteria growing in it. Very easy to just drop in a pipe or hose for the pump and I used duct tape to seal around mine. I had a second pipe to fill it with and I left that open so I would not create a vacume when pumping out the water, just put a filter over it. These worked very well for me winter and summer, though I did melt the first one on a hot sunny summer day in direct sunlight. Half or less full of water and covered with black plastic it got so hot the top of the sides bent all goofey out of shape. Solar heated water in summer?
 For filling the pool or other type cisterns, $6 for a stock tank waterer. It is plastic with a float, mount to side of pool connect hose turn on water, when the pool is full the float shuts off the water from the hose. This may be good to have, I had several hot water tanks piped together and gravity feed into pool when hauling water, this stopped me from over filling my pool and running out extra water. I just connected the hose, opened the valve and walked away, let gravity fill the pool.
 Later I used it on my hog water tanks after I got a well, they shut off good even with presure! For filling yours up in the winter you won't want water over flowing everywhere making lots of ice, and you may not want to stand around watching it constantly either in -10 or below. It may be nice to have one in case the pump runs  a few minutes too long.

Another tank I used and still do is a 220 gallon white plastic tank, I think it would freeze and bust up there though and they aren't cheap!

Good unsented CLOROX bleach, not generic, added to the water once in a while should keep it from growing anything and kill any bacteria that may be in the river water. You don't need alot! Been many years, but I think it was less than a cup for over 100gal of water. Maybe only 1/4 cup? All I remember was it was very little. Should be able to look it up online somewhere. The city water guy told me the ratio when I needed to do that but I forgot now. After you bleach the water you can leave the tank open and the bleach smell will evaporate. Not sure if all the bleach evaporates ( I think it does) but at least the smell does for sure. You don't need alot like a swimming pool, your not going to be swimming or bathing in it :)
Alot of people want to use way too much bleach, thinking of swimming pools I geuss.
 Also some generics have extra garbage in them, so you want pure clorax.

For drinking and cooking water you won't need nearly as much, It might be good if you   had maybe a 35gal barrel if you have room in the cabin for it. You could do a siphon or maybe build a rack so it is overhead, build in shelfs or sink under it. Kinda like Clydes shower but indoors and make use of the area under it so space is not wasted.

 How much water are you wanting? It is a pain to flush the sediment out, normally the only thing wrong with the hot water tanks I get free or $3. Then I bleach them also.
When I hauled water I had the tanks piped together at the bottom and a shutoff valve at the end of the pipe where I would connect the hose at the house to fill the pool. Water seeks it's own level and with all the tanks piped together I could fill one tank and that filled them all, I left the top pipe nipple uncapped on all the tanks.
 If you were to do similair you could run a pump on the end of the bottom pipe and get a feed from all tanks at once. This would not take up too much room indoors along a wall. Maybe 2-3' round tanks about 5' tall, 4 along a wall, 150-200gal of water storage. Or you could lay 35gal barrels on their sides stacked high in a rack, run a pipe from the top one to the one under it connecting them each to the next one down. Fill the top one and they all fill, just series them :)
 If you want more water than you can stack barrels high, make 2 series stacks and paralel them at the bottom pipe, leave both stacks with an open hole at top to prevent vacum when draining. :)
 $6 each here for 35gal plastic barrels, 10/$60 and alittle pipe and few fittings for 350gal storage, plus build a rack for them.

 Again we don't have the severe cold here you have there but I have left those barrels in the yard full of water all winter (when we did have cold winters) and never broke a barrel here yet, the pipe would break before the barrels I am sure, and that would make a big mess. But they would be easy to drain if your not using them for extended times.

 A nice thing about tanks of water indoors, they will warm to room tempature eventually, if you leave for awhile and have no fire they will radiate that heat back into the cabin helping it stay warm longer.

We have been having too warm of winters, yesterday and today was like spring time. So I have not built the solar heaters I was going to build, it just hasn't been cold much to need them. We used to have cold winters and snow, 1 small snow so far 1 day, a bit before Christmas. I been making a workshop out of the old trailer house I use for storage, the one we lived in when I hauled water. The well is up at house so I have to run a water hose to the shop several hundred feet (maybe 300' or more). I was going to use the 35gal barrels in the shop for water storage and heat storage like I mentioned, but I was going to stand them up right like normal and build a workbench over mine. Then put a solar heater outside at ground level and let the heat rise up into the trailer and around the barrels durring the day. Depending on the amount of sun this would warm the barrels of water and the trailer some. Being the solar heater would be below the floor level of the trailer I should not have reverse convection when things cool at night, no reason for warm air to flow downwards or cold air to rise. Barrels of water would also warm when I have a wood burner fired up for heat in the shop, then keep it warm longer over night, and also warmed water for use.
 If your cabin/house is on piers and you get good sun that may be an idea to think along with the water storage. It would have worked for me, but like today I was thinking about opening the house door and turning on the fan, did not need extra heat today. Storing a bit of heat in water tanks would have been nice, 4am-7am only about 60F in the house, no need to build a fire since the place will warm up in a bit when the sun pops out. 350gal of water would probably kept the house around 65F over night maybe?
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nothing to lose

The propane people expressed ill-will toward me many a time for my habit of heating throughout the down to -40 degree Fahrenheit (also Centigrade) winters there with nothing but the wood I collected (with much time and effort) from the woods in back (never cutting more than the dead trees there) while my propane tank remained half filled and unused throughout.

On one especially cold day I couldn't find any but just a small opening in the ice over the river at the rapids and filled my plastic bucket there before the long trek back to my residence. I heard the usual two toned whistle of the little bird that overwinters in most of Minnesota and out of curiosity went to try to see him to identify him. Later I found these are the black capped chickadees, such a hardy small bird. When I got back, my bucket had completely frozen top to bottom. Such is life where energy for heating is such a necessity.

If you can pump and filter your water when you need it, then it might not hurt to drain your system when not in use.