Yet another element of a fly line is its density - which basically means whether it floats, sinks or only partially floats. For most angling conditions, a floating fly line is what is used and is by far the most popular. While the fly line itself will float, using nymphs or attaching weights will pull down the leader, allowing adequate sub-surface fly fishing in most conditions. If you can only own one fly line, make sure it is a floating line.
A sinking fly line is used mainly in big, slow moving rivers or in lakes. As the entire line sinks, it can take a fly down to very deep depths quite quickly (depending on its sink rating). Since the fly line sinks, a sinking line is difficult to pick up and can be a bear to cast, especially on a long day.
There are several flavors of sinking lines available. The intermediate sinking fly line sinks slowly and at a uniform rate and is a great choice for fishing on lakes that have lots of weeds. A full sinking fly line will completely sink at a uniform rate. How fast the line sinks will vary depending on the actual lines sink rate, which is usually between 2-10 inches per second. Last is the fast-sinking fly line which sinks like a stone. This type of fly line is really only needed for saltwater fly fishing or when fly fishing deep lakes.
Finally, sink tip fly lines are a combination of both of the above. On a sink tip line, the first 10-30 feet of the fly line sinks while the remainder of the line floats. This greatly assists in line pickup and casting, while still allowing the line to pull down flies to deep depths quickly.
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