Main Channel Length!

[Siavash Bassam]

If you are using auto watershed delineation, the length of some reaches are less than the actual length of the main channel. i know we can later change the length of the main channel. What would happen if we do that? The shape of the reaches will be the same, so why SWAT let us change the length of the main channel? 

Also, what should be the length of tributary channels? thee default is equal to the length of the main channel. Should we estimate the length of tributary channels in each watershed, then use it as the input?  

Thanks

Sia