What a critical source area is on your map

A critical source area, or CSA, is a landscape feature such as a gully, swale or depression that collects runoff from the land around it and can carry nutrients and sediment to a river, lake, wetland or drain. Beef+Lamb New Zealand's Farm Plan Environment module defines it this way, and the definition holds whether or not there is any water sitting in the feature when you look at it. A dry gully on a sunny day is still a critical source area if it floods and runs during heavy rain.

What exactly makes a feature a critical source area?

Three things have to line up: a shape that accumulates runoff, a connection to land upslope that can shed water, and a path, however indirect, to a waterbody or drain. Beef+Lamb New Zealand's guidance names the shape explicitly: "a gully, swale, or depression that accumulates runoff from adjacent land and delivers, or has the potential to deliver, one or more contaminants to one or more rivers, lakes, wetlands, or drains, or their beds." The phrase "has the potential to deliver" matters. You do not need to see contaminants moving through the feature to mark it as one. The shape and the connection are enough.

This is why the same source lists critical source areas and overland flow pathways under the water features group on a farm map, alongside waterways, drains, constructed water features and water crossing points. A CSA is treated as a water feature even on days it carries no water at all.

How do critical source areas differ from the waterways already on my map?

A waterway is a feature with a channel: a stream, a ditch, a drain with a defined bed. A critical source area is often upstream of any channel, and may have no visible channel of its own. It is the collecting ground, not the outlet. What a base map actually shows already asks you to record troughs, dams and waterways on your map's water row; a critical source area belongs on that same row, but it is the part most base maps miss, because it can look like ordinary pasture on a dry day.

Feature	Typical shape	Carries water	Easy to miss
Waterway	Defined channel or bed	Usually, at least seasonally	Rarely
Drain	Constructed channel	Often, by design	Rarely
Critical source area	Gully, swale or depression	Only during and after rain	Often, especially when dry

Where on the farm should I expect to find one?

Look at the land, not the paddock boundary. Critical source areas sit wherever slope and shape concentrate runoff: the low point of a long paddock, a swale running between two ridges, a depression below a track or raceway, a gully that only shows standing water after a storm. Landforms worth separating sets out which landform differences are worth distinguishing on a map in the first place; a gully or swale that behaves like a critical source area is one of the clearest cases, because the shape changes what the land does during rain, not just how it looks on a calm day.

Slope matters here too, but not in isolation. Reading slope and aspect shading explains how steepness and direction change drying and sun exposure; a critical source area adds a third question on top of those two, which is where the water that falls on that slope actually ends up collecting.

Why mark a dry-looking feature on the map at all?

Because the risk a critical source area carries is about timing, not appearance. The feature can sit dry and grassed for most of the year and still deliver a meaningful load of sediment or nutrient during one heavy rain event. If it is not marked, nobody plans around it: stock get moved across it without a second thought, fertiliser gets applied right up to its edge, and the gully only gets noticed as a problem after water has already reached a drain or stream. Marking it in advance turns a surprise into a known constraint you can manage on your own terms, such as adjusting the timing of nutrient application nearby or fencing it separately from the rest of the paddock.

How do I record one so it survives a staff change?

Treat it the same way you would treat any other feature worth keeping track of: a short, plain description plus its location. Note what it drains towards, roughly how large its catchment is, and whether it has ever visibly run during a storm you remember. You do not need professional survey detail to start; a hand annotation on your working map, with a one-line note in your companion record, is enough to stop the feature being quietly forgotten.

If you are already building land management units on your farm, a critical source area is usually not a unit of its own; it is a feature that sits inside one and changes how that unit should be managed. The resource chart used to describe a unit's strengths, weaknesses and conditions of use is the natural place to note that a given block contains one or more critical source areas, and what that means for nutrient timing, stock access or fencing priority within that unit.

The short answer

A critical source area is a gully, swale or depression that can carry nutrients and sediment to a waterway when it rains, whether or not it is carrying water the day you look at it. It belongs on your map as a water feature, next to the waterways, drains and crossings you already record, because the risk it carries is about what happens during a storm, not what the ground looks like on an ordinary afternoon. Find the low points and depressions that collect runoff from the land above them, mark them once, and you no longer have to rediscover them after the fact.