It is now accepted here in Australia and I am sure in some land management circles in NZ, that piping or concreting natural waterways creates significant environmental problems. This is now recognised in most developed countries. By concreting a waterway, the velocity and volume of the water is increased. I wo'nt go into the figures, but as communities expand, there is a incremental increase in the surface run-off of water.

Imagine the inner city area of say Auckland. Most of the land area is now covered with buildings, roads, etc. so that nearly all the rainwater just runs off into the drainage system. The more the suburban fringe expands and the inner area gets built on, the larger the volume of water, that is prevented from soaking into the soil or just meandering through a catchment. But it all starts upstream, when the first house gets built and then the roads and so on, etc. etc.

Back to the creek. When increased runoff hits a nice smooth concrete surface it gets faster and compounds to such an extent, that severe erosion and flooding now occurs down steam. This can be disastrous.

In my landscaping lectures I recall seeing lots of photos of erosion and slippage etc. in New Zealand. Flooding is now tragically common in the lower suburban areas of the Sydney basin and I am told in parts of Wellington. It may well occur in other communities of New Zealand as well.

As a result there are severe requirements on housing estates etc. to inhibit water runoff. Swales, detention basins(dams), wetlands to take out nutrients, etc.,are now mandatory. You will find this the same in America and Europe as well.

For example, those big open concrete drains we see beside freeways that once were creeks are now being ripped up. Landscaping companies are being contracted to reinstall a sort of creekline that has curves etc to slow the water. Millions are being spent here, to recreate a sort of creekline with obstructions like rocks and even the broken concrete from the old open drain is being used. Vegetation is being reintroduced as it also slows down the water as well as helping to deal with erosion, take out nutrients, encourage habitat etc.

What appear to be large parklands or football fields, are now detention basins designed to hold water run-off in severe downpours.

Regarding your problem with the eels. This is an area where I know very little about. But I would suggest introducing more rocks of varying size so the holes are smaller between them. Is there a way to live with the eels and yet prevent them damaging the creekline.

My bet is, if you put down a pipe you will add to a problem further down stream. A natural stream sounds like a great thing to have. Consider the cost piping the creekline and spend that budget on planting and improving the 'natural' look.

Might I dare suggest you talk to a sandal 'wearer.' One of them might just be of help regarding the eels.

P.S. Sandal wearers of the world, unite!!