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What really causes erosion?
The attached pictures were take on Sunday 8 August 2010 and emailed to me couple of days ago. They were taken by a walker, walking round the walking track at Manly Dam.
Manly Dam is one of the few places in the area with legal single track and as you would expect, this means that there is no evidence of riders being tempted to go elsewhere (when legal riding is supplied there is no reason to ride illegally). IE. there is no evidence of any riding on these tracks from the pictures.
Now, why this is interesting is that opponents of mountain bike riding are always trotting out the line, "Mountain bike riding causes erosion!".
By the same logic, looking at these images one would have to conclude that walking causes erosion too - and much more than riding does as there is no-where on the MTB circuit in this bad a state!
The plain truth is that any activity causes erosion to some degree but what is much more powerful is what water can do - just look at the images after all the heavy rain we've had this winter.
These pictures speak to that: no matter what activity a track is meant for, if it's poorly designed and doesn't handle water flow properly there will be problems.
They also show that maintenance is needed on all types of track, those for walking and riding alike. And that mountain bikers aren't alone in having their facilities neglected by local authorities 
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After those prolonged heavy rains we had in February, the Southern Cross Drive trail section at Manly Dam - where council spent a small fortune putting in those across-trail drains - showed ample evidence of what the primary erosion driver is: water flow.
It was very obvious which sections riders used heavily, and it wasn't where the damage occurred. The obvious compacted line was on the opposite side of the trail, about a metre distant from the vegetation.
It was even more obvious on the fire trail that comes down from behind the Lutheran homes. Huge erosion gullies on the low side, with the obvious trail favoured by the majority of riders evidenced by compacted line on the opposite side for most of the length of damage. The only exceptions were where the erosion gully ran across the bike trail towards the bottom of the slope.
The water flow problems are caused by poor trail routing:
* too steep an incline
* running the trail straight down slope
* letting the trail run too long before reversing the incline
* not sloping the trail to the side to spill water off the trail
* routing the trail through sections with poor drainage
* where the above are unavoidable, failing to put in appropriate trail armouring measures (elevated bridges, effective drains, rock armouring etc)
Anyone care to count how many of the above trail mistakes can be seen in each of these photos?
The paradox about not being able to ride in National Parks but being allowed to ride in reserves is that the trails in National Parks tend to be better designed and managed. After all, whilst I am not saying NPWS have massive resources, they employ rangers whose primary purpose is to look after that park as opposed to council rangers who do not have the reserves as their primary purpose.
I have just returned from Cascades where the NPWS rangers were on the trail stopping at each water bar and making sure that they were both draining and also not becoming eroded. This meant that water was flowing across the trail rather than sitting there eroding the trail and then flowing down its length. I do not think that places like Manly Dam simply do not get this sort of attention.
One would think that it makes sense to allow activities throughout the places that better designed and managed but this is just wishful thinking.
Rockytrail Entertainment fairies up your way guys ?
Trail fairies abound in these parts. Sadly... their pixie dust is not allowed by council. That's the whole problem
Another walking track that follows the water course.
I predict it will be decked it out in 20k worth of treated timber within the next 2 years.