THE LEFT FORK OF NORTH CREEK: Overview of Various Routes
The Left Fork of North Creek (aka "the Great West Canyon") is one of the most beautiful and most popular canyons in the backcountry of Zion. The most famous section of this canyon is known as "The Subway" -- an amazing half-mile stretch that resembles a tubular tunnel complete with tracks going down the middle. The Left Fork of North Creek has much diverse terrain, and as such, there are several different possible routes, ranging from simple hiking to strenuous technical canyoneering.
Wilderness Permits Are Required:
The Left Fork has gained epic popularity over the past several years and the National Park Service now limits access to the Subway to 80 people a day. Every route through the Left Fork requires you to obtain a permit, technical canyoneering or not. Because the Subway is so popular, the NPS also has a lottery system in place to manage the distribution of permit reservations. This layer of bureaucracy may be a bit tedious, but if you plan your hike well in advance, your chances are pretty good for getting a permit for a day that you want. A limited amount of permits are also available through the last-minute lottery drawings. Please see the Zion Permits website for more information. Policies and procedures do change over the years.
Route Options:
There are several possible variations for hiking through the Left Fork; all are day-use routes as camping is not permitted in the Left Fork. The "bottom-up" route is the only non-technical option. 
- Subway Top-Down Route (via Russell Gulch):
This is the classic way to hike the Left Fork. Starting from the Wildcat Canyon Trailhead, hike down Russell Gulch to join the Left Fork and hike down through the Subway. This route involves several rappels and several cold swims and is not suitable for hikers who have no technical canyoneering experience or gear.
Canyoneering Guide Photos - Subway from the Bottom (Non-Technical Hike):
If you want to avoid the swimming and rappelling, you can start at the Left Fork Trailhead and hike to the Subway from the bottom. The scenery is less stunning, but you still get to see the beautiful lower section of the Subway.
Hiking Guide Photos - Descent of Russell Gulch:
Similar to route 1, but instead of following the hiker-friendly route down Russell Gulch, you can descend the Russell Gulch watercourse which involves a few 100' big-wall rappels in the belly of the canyon. Note: This canyoneering route requires a separate permit and is a much more serious endeavor than the standard top-down route.
Canyoneering Guide Photos - The Das Boot Section:
"Das Boot" is the humorously-named half-mile section of the Left Fork upstream from the standard Top-Down Route. This technical canyoneering route involves several swims, rappels, down-climbs, and exposure to year-round cold water and is not suitable for casual hikers. Note: This canyoneering route requires a separate permit.
Canyoneering Guide Photos - Top-Down from Wildcat Canyon:
A long and difficult canyoneering route starting from Wildcat Canyon at the headwaters of the Left Fork.
Seasons and Conditions:
The hiking season for the Subway is typically late spring through autumn, but conditions are much more difficult during the higher water conditions during spring runoff which usually takes place during April and May. During high water conditions, spots like the bowling ball corridor and Keyhole Falls can be impassible and other sections may be treacherous. 
During the cooler months, a wetsuit may provide welcome comfort for those doing the top-down route. If you have any concerns about conditions, please ask rangers at the Wilderness Desk to get the latest information. The top-down route is not advised during the winter months and the Kolob Terrace Road is typically closed and buried under snow a mile before the Wildcat Canyon Trailhead.
Note on Commercial Canyon Guiding:
Many people who have no canyoneering experience want to do the Top-Down Subway route. Unfortunately, the National Park Service does not permit commercial guiding within the boundaries of Zion National Park. Several outfitters in Springdale do offer canyoneering training, equipment rental, and guided canyoneering outside of the park boundaries. See the bottom of the Zion Hiking page for more information. Alternatively, if you have access to a local climbing gym, getting firsthand experience learning to rappel properly in a climbing harness would be very useful.
