This started out as a single post, but I just kept rambling, so I’ll be dividing it into 2 parts: a background on erosion in general, and gullies in particular, followed by part 2, actions that we’re doing here on the farm to mitigate their impacts. Comments are appreciated
A gully is a response that land makes to erosion, either up or downstream of it. Upstream, flows get concentrated and speed up, and when they reach a softer point, the begin to wash away soil, dropping the level of the stream bed. This drop increases turbulence, which can increase the amount of soil washed away. As the erosion continues, the gully “walks” upstream, as it’s base gets continually eroded away.
(image from USFS)
Downstream actions can also cause gullies. When a channel is dug out (for example, in the deepening of a pond), it can create a drop location, or nickpoint. If the soil there is soft, it will create a gully that will run upstream until a balance is reached between flows and soil integrity. Often, this balance is reached in the form of an incised channel. The water level is lower than it was previously, and the soil doesn’t store as much water as it once did.
When we moved here, our dog Boomer fell into a gully created by these erosion events. We were walking through some brush, and he pretty much vanished with a yelp. After I pulled him out, I found the hole created by the gully over 6′ deep. As I got to know the farm better, I found more of these sites (but none as spectacular), usually just downstream of every place where a fence crossed a channel.
These gullies create a problem for a dryland farm such as ours. We are entirely reliant on a combination of rainfall and groundwater. While our rainfall is around 50″ per year, it mostly falls when it’s not needed, from Oct – May. The groundwater component of our water is what carries us through the drier June-Sept period, when the temperatures are warmer and most of our region’s productivity is expressed.
(image from USFS)
For more information, these links can give more background
http://www.fs.fed.us/t-d/pubs/htmlpubs/htm06232815/page30.htm
http://www.fao.org/docrep/006/ad082e/AD082e00.htm
http://www.nre.vic.gov.au/dpi/vro/vrosite.nsf/pages/lwm_land_deg_gully_form?OpenDocument
Part 2 runs down what we’ve done to address some of our gullies.