A run of river hydro project that dewaters this whole section of river came online in 2016. While this has drastically changed the runnable season, there is now an online gauge and 4 release days each year. The head pond does not impact the popular Upper Big Silver run.
There is an accurate online gauge
for the Middle Big Silver, making the long drive to get there more palatable. We don't know what flow is best for this section. Innergex provides up to 4 days of releases per year (in July and August) at a flow of 15 cms, which seems like it would be a good flow. You need to sign up, similar to the Ashlu system, for a release to happen. It should also be possible to run this section when there is some water spilling over the dam during peak snowmelt. The season for this run before the dam was in late summer and early fall.
Go to Harrison Hot Springs and take the Harrison East FSR up the east side of Harrison Lake. The kilometers are well marked the whole way. When you reach Big Silver Creek, the Harrison East FSR will begin to head up the Big Silver Valley. The reference point for the Middle canyon is where the big bridge on the Harrison East FSR crosses from river left to river right at approximately km 47 (give or take). The put in is at the dam upstream from the bridge. The take out is downstream of the bridge at the power house.
The Middle Big Silver is a serious stretch of whitewater in a committing granite gorge. The easier half of the run is upstream of the bridge where it seems straightforward. Just below the bridge as the river bends left is the crux of the run. Get out here if you have to as it will be difficult or impossible downstream. Several class V rapids lead in to a very marginal waterfall that to my knowledge hasn't been run. The portage looks horrible, and you should scout the gorge before you commit to make sure you want to deal with this drop and the portage. More rapids lead to a triple falls. The first two also look marginal and haven't been run (again, as far as I know) - portage and run the last one. It's a short paddle to easy water and the take out at the power house.
Keep in mind this river beta is from an extensive scout, not a descent. We deemed it was too much dealing to run just a few good rapids.