Topobathy: One Seamless Elevation Map of the Upper Mississippi River's Land and Riverbed

The Upper Mississippi River System — the navigable parts of the upper Mississippi and Illinois Rivers — is a varied ecosystem of river channels, tributaries, shallow wetlands, backwater lakes and floodplain forests, shaped by about 10,000 years of geological and hydrological history. It supports vital habitat for fish and wildlife, and its ecosystems depend on the neighbouring floodplains and on the rise and fall of the river.

The U.S. Army Corps of Engineers' Upper Mississippi River Restoration Program collected two separate sets of elevation data along the system: the height of the floodplain, by lidar, and the height of the riverbed, by bathymetry. From the two, the U.S. Geological Survey's Upper Midwest Environmental Sciences Center built a single, system-wide topobathy dataset.

The Upper Mississippi River System and the restoration programme's area, where topobathy is available, across Minnesota, Wisconsin, Iowa, Illinois and Missouri. USGS.

What topobathy is

Topobathy is a seamless surface model of elevation on land and under water.

• On land, elevation comes from airborne lidar (light detection and ranging), a pulsed-laser method of measuring the Earth's surface, turned into digital elevation models.
• Under water, riverbed elevation is calculated from bathymetry, depth measured with sound (hydroacoustics).

Combine the two and you get one continuous surface.

Bathymetry and lidar combined into seamless topobathy. USGS.

The data behind it

From 2007 to 2011 the restoration programme flew lidar of the whole system from bluff to bluff at one-metre resolution. Lidar from Minnesota, Missouri and Illinois, and further data from the USGS and the Corps, filled the remaining gaps. The products — one-metre elevation models and hillshades, and half-metre contours — were all processed by 2015.

The Upper Midwest Environmental Sciences Center and the programme's Long Term Resource Monitoring element have managed the system's bathymetric data since 1989; system-wide surveying was completed in 2010. Applying measured water-surface elevations at chosen flows turns water depth into riverbed elevation. Bathymetry also serves to monitor the river's ecology, study sedimentation, assess the effects of changes in flow, and model where vegetation grows.

What it is for

Researchers, scientists, governments and private companies are turning to topobathy as a vital elevation dataset. On the Upper Mississippi it is a key layer for understanding, monitoring and restoring the floodplain and the river, and it can feed floodplain and river models, research, resource monitoring and mapping.

• Better flow models. Until recently, hydrodynamic models could predict only low-water conditions; with topobathy, they can be built for many different flows.
• The Connectivity and Inundation tool, a decision support system developed at the centre, produces map layers summarising water conditions over a chosen date range, to show the combined effect of changing flow and depth.
• Filling the gap at the shore. Near-shore areas are almost impossible to measure: airborne lidar cannot see through murky water or dense vegetation, and survey boats cannot reach the shallow edges, leaving a thin gap along the banks. Interpolating between the boat survey and the shoreline lidar gives a predicted surface for these areas — a continuous record of near-shore elevation that managers and researchers have not had before, and one that may matter for judging how water-level management affects the transition zones of the floodplain.

Topobathy of Mississippi River Navigation Pool 8, south of La Crosse, Wisconsin: a braided channel and its backwaters in three dimensions. USGS.

The Corps of Engineers' Weaver Bottoms restoration project, south of Weaver, Minnesota, on Navigation Pool 5, in topobathy. USGS.

What comes next

Topobathy is one of the fastest-growing geospatial products for riverbed, near-shore and floodplain habitats. Topobathymetric lidar, a high-resolution sensor using green lasers, can measure land and water on the same day with a single instrument, cutting the cost of collection. It has limits, though: it cannot see far into murky or weedy water, its depth range is restricted, and it depends on the weather. The Upper Mississippi is generally too turbid for it today, so sound remains the best way to measure the riverbed. As the technology improves, the whole system may one day be mapped from the air alone. Meanwhile, a plan is being drawn up to replace the outdated lidar and bathymetry in parts of the dataset.

The lidar and topobathy data are free to download from the centre's data library.

Sources

• U.S. Geological Survey, Upper Midwest Environmental Sciences Center (contacts Jayme M. Stone, Jenny L. Hanson and Stephanie R. Sattler), The Upper Mississippi River System—Topobathy, USGS Fact Sheet 2016–3097, March 2017. https://doi.org/10.3133/fs20163097
• Data: http://www.umesc.usgs.gov/data_library/topobathy.html