It doesn't have to be very efficient, but it is has to be cheap. Batteries aren't cheap but they discharge every night so their cost is spread out over ~365 days and their cost becomes feasible.
Pumped storage is ~10% the cost of batteries, but if used as seasonal storage so they only discharge once per year that makes them ~36X as expensive. Especially in Alberta where they're competing against Natural Gas which is also approximately free (it's often flared off as a waste gas from oil wells rather than captured and sold because the value is less than the cost of capture).
For seasonal storage, it would be either an e-fuel (like hydrogen) or bulk storage of thermal energy (basically, artificial geothermal). The latter could produce very hot rock at rather shallow depth, yet still have thermal time constants of many years.
Hydrogen on this scale would be stored underground, preferably in solution mined salt caverns. This is a well established technology, although it's more used for storing natural gas. These caverns are very inexpensive to build, which is why they are used.
Alberta, as it turns out, is underlaid by kilometers of sedimentary rock that includes salt formations.
If one compares the heat of combustion of hydrogen stored at 100 bar and 20 C, to the energy of water elevated by 100 meters, the former exceeds the latter (per unit volume) by a factor of 1000. So that "holding pond" is going to be much, MUCH larger than the solution-mined cavern for storage systems of equal capacity (and remember one needs two reservoirs.)
Pumped storage is ~10% the cost of batteries, but if used as seasonal storage so they only discharge once per year that makes them ~36X as expensive. Especially in Alberta where they're competing against Natural Gas which is also approximately free (it's often flared off as a waste gas from oil wells rather than captured and sold because the value is less than the cost of capture).