River crossings on forestry sites
River crossings are a common part of forestry roading. They enable access across waterways to support activities like planting and harvesting.
Good planning and design of river crossings protects your road network, reduces environmental impacts, and keeps your operations running safely and efficiently.
This page includes points you’ll need to consider when planning river crossings at your site. It doesn’t cover regulatory requirements or technical information about earthworks.
Quick tip
Choose the safest, most stable place to cross a waterway, and build the crossing well so it protects the environment and your road network.
Why river crossings matter
A river crossing is a structure that allows vehicles or machinery to cross a waterway. They provide access for activities such as planting, harvesting, and transporting logs.
You might need river crossings to reach different parts of the forest. Without safe crossings, machinery and trucks might not get to harvest areas, landings, or new road sections.
Common examples of river crossings include fords, culverts, drift decks, temporary log crossings and single span bridges. They include crossings used to access a forest, even if they’re not located within the forest itself.
A well-planned and well-built crossing has:
- good alignment with the road or track
- stable approaches and “fills” (the material used to build-up the approach)
- a clear, unobstructed water flow through or under the crossing
- a design that allows for peak water flows based on catchment features such as size, slope, rainfall, and upstream land use.
Indicators of a poor crossing include misaligned approaches, an unstable structure, blocked culverts, and build-up of erosion and debris. Problems become more obvious after heavy rain.
Types of river crossings
The type of crossing you need depends on:
- how much water flows through the site
- how often you’ll use the crossing
- whether the road is temporary or part of long term access
- how difficult the site is to build on
- the level of risk if the crossing fails
- the machines or vehicles that need to use the crossing.
Here are some of the crossing types found in New Zealand.
Fords allow vehicles to drive through streams or shallow rivers. You might use a ford where flows are low and infrequent access is needed. Because vehicles pass directly through the water, they can cause sediment disturbance during use. They work best in streams with stable beds and where sediment can be effectively managed.
Culverts allow water to pass through a pipe or box structure beneath a road or track. There are different types, including single culverts and battery culverts. Well-designed culverts let water, sediment, debris, and fish move through them without causing erosion, blockage, or flooding.
Drift decks are low‑level structures that allow water to pass underneath during normal flows and over the top during high flows. During floods, they’re designed to let water and debris pass through.
Single span bridges cross waterways and allow water, sediment, debris, and fish to pass underneath. They’re typically used for larger streams or where environmental risk is higher. Bridges can reduce disturbance to the streambed but need careful design to handle high flows and debris.
Removable in-stream structures sit in the stream for a limited time, but longer than temporary crossings (see below). They’re designed to allow low flows to pass through and high flows and debris to pass over the structure.
Existing river crossings already in use may not meet all current design standards. However, they’re generally allowed because upgrading or maintaining an existing crossing can have less impact than removing and rebuilding it. You need to maintain existing crossings, so they function properly and don’t cause environmental harm.
Temporary crossings
Temporary crossings include culvert and log structures sitting in the waterway’s flow path or log bridges that span it. They’re used for short-term access to the site during harvesting or other activities.
These crossings can provide flexibility in planning. They allow operations to respond to weather conditions. For example, you can assemble them for work during dry periods and remove them before forecasted heavy rain.
Temporary crossings still need careful planning. If they fail, they can cause the same problems as permanent structures, including erosion and sediment entering waterways.
When crossings are built
You typically build river crossings:
- when you construct new forest roads
- before harvest, so machinery can reach landings
- during forest development or maintenance.
It’s best to build crossings during dry conditions. This reduces sediment entering waterways, improves ground conditions for machinery, and lowers the risk of damage during construction. If you only need access for a short time, consider temporary or removable crossings to reduce impact on the waterway.
Planning crossings early reduces delays and helps match the structure to your long-term road layout.
Planning a river crossing
Planning is where you have the biggest influence on cost, safety, and environmental outcomes. Investing time and money on proper planning can reduce ongoing long-term costs. A poorly designed crossing can lead to repeated repairs, hold-ups to forestry activities, and even full replacement after structural failure. Complex or permanent crossings usually need input from an experienced designer or engineer.
Here are some points to consider.
Minimise the number of crossings
Crossings are expensive and increase environmental risk. Keep them to the minimum needed for safe operational access.
Choose the safest place to cross
Look for the most stable crossing point with good access on both sides. Aim to cross the waterway as close to perpendicular (right angles) as possible so as not to redirect flow. Avoid steep banks, unstable soils and tight bends.
Think about how the crossing fits into your wider road network and harvesting plan. This makes access to your forest practical, efficient, and easier to build and maintain.
Understand water flows
Check typical flows, peak flows, flood history, and how debris moves through the waterway, especially during storms. Even small streams can carry large volumes of water and debris during heavy rainfall. You need to design crossings for high flow events as well as normal conditions, so they don’t collapse or cause ponding or erosion.
As well as past floods, plan for more frequent extreme weather events. A resilient crossing must stand up to "one-in-100-year" storms that occur more often.
Consider how water behaves across the whole catchment, not just at the crossing point. Features like catchment size, slope, rainfall, and upstream land use all affect how quickly flows rise and how much force or debris a crossing needs to handle.
There are several tools and methods to estimate the size of floods a crossing needs to withstand:
Consider fish habitat
Some waterways support fish that need open passage to move upstream and downstream. Designing crossings so water, sediment, and fish can move naturally supports healthy freshwater systems.
The flow of water through a crossing influences fish passage. Structures that create sudden drops, high water speeds, or smooth, bare surfaces can make it difficult for fish to move through, especially during low flows. Crossings can become barriers over time if the outlet becomes “perched” above the streambed or if sediment builds up.
To improve fish passage, crossings should:
- follow the natural alignment and gradient of the stream
- maintain or mimic the natural streambed inside the structure (for example, by embedding culverts and allowing natural material to build up)
- avoid creating drops or steps at the outlet
- limit water velocity, so fish can move through at a range of flows
- be kept clear of debris or blockages that restrict movement.
In some locations, you might need to restrict fish passage to protect native species from invasive fish. Check with your local council or relevant authority to understand the needs for your site.
Think about long term maintenance
Crossings need ongoing checks for debris, erosion, and wear. Planning for maintenance reduces failures and protects your road network.
Regular inspections, especially before or after heavy rain and floods, help identify issues early. Look for signs such as:
- debris build up at inlets or around structures
- erosion or scouring around outlets and abutments
- damage to protective materials
- changes in water flow through the crossing.
Addressing small issues early, such as clearing blockages or repairing erosion, can prevent larger failures and reduce long term repair costs.
Risks of river crossings
River crossings can create environmental and operational risks – such as structural failure – if they’re not well planned, built or maintained. High flows and storms increase the risks.
Specific risks of poorly managed crossings, include:
- sediment entering waterways
- blocking or disrupting fish passage
- structural failure
- erosion of banks and approaches
- debris build up around culverts and bridges
- damming or erosion if crossings are damaged during storms or floods
- safety risks of people and machinery.
River crossings common terminology
The New Zealand Forest Road Engineering Manual includes some terms you may come across when planning river crossings.
Terms used in this article include:
- Approach: Entry and exit sections of the road on either side of the river crossing.
- Abutments: Structures at each end of a bridge that support the deck and hold in place the fill of the approaches.
- Fills: Layers of soil or aggregate used to build up and support the approaches to a river crossing.
River crossings guidance
You can read more detailed, practical guidance from industry sources. These guides give you more context and help you talk with engineers or contractors.
The NZ Forest Road Engineering Manual covers types, design considerations, and construction basics for crossings.
The Forest Practice Guides explain environmental risks and best practice methods for managing effects from crossings.
River crossings are a regulated activity
This page does not cover regulatory requirements for river crossings. Building and maintaining river crossings on commercial forestry sites are regulated activities. You’ll need to comply with the National Environments Standards for Commercial Forestry (NES-CF) when constructing river crossings. You may also have health and safety responsibilities for forestry activities on your site.