Temporal analysis: Historical
Historical approaches to the temporal characterisation of hydromorphology
Historical approaches examine the human record and enable changes in hydromorphological processes and forms to be estimated or quantified. These techniques rely on documentary evidence (diaries, deeds, etc.); land and tax surveys (i.e. cadastral surveys and maps); historical maps; river topographic surveys (e.g. repeated longitudinal profiles and cross sections of river channels) and terrestrial photography. For an introduction to the use of historical data in fluvial geomorphology, see Gurnell et al. (2003) and Trimble (2012)
The historical approach is applicable to all spatial scales. Due to the diversity of data sources included in this category, it is applicable to a wide range of timescales. In reality, though, the use of historical evidence is severely limited by the availability of data sources for a particularly location or time period, the type of data that is available (e.g. observations or scientific measurements), and its reliability or accuracy.
Historical sources should be carefully screened before inclusion in a study of temporal change (Hooke and Kain, 1982). First an internal check of the data source should be conducted to ascertain the purpose of the source, when information was observed and subsequently published, whether it was an original survey or revision, who the observer or reporter was, what methods or instruments were used, and, for surveys and scientific data, what were the reported levels of accuracy. Second, additional sources should be used to corroborate the primary source, verifying its spatial accuracy (e.g. specific features were in the correct geographical location), attribute accuracy (e.g. features were identified correctly) and its temporal accuracy (e.g. information is correct for the reported date). As with remotely-sensed data, it is very important that accuracy be assessed so that genuine spatial and temporal changes can be differentiated from those that are artefacts of the data collection, interpretation, representation, storage or digitisation.
Some historical sources, like discharge records, offer precise scientifically-derived datasets with daily or even instantaneous measurements, but most historical sources provide widely-spaced or individual snapshots of hydromorphological characteristics. For example, large-scale maps provide researchers with a variety of valuable information (e.g. channel dimensions, planform, land cover, floodplain and channel geomorphological units), but mapped features are subject to surveyor interpretation and modification for cartographic purposes, and revisions may be spaced decades apart, especially in remote or rural areas. Topographic surveys can be very valuable sources of information, but they are commonly available for only the largest streams or for streams crossing populated areas. Documentary evidence is even more limited in its utility for interpreting historical changes since it often describes only a single point in time or space, and its accuracy / precision is rarely defined. Therefore, this type of evidence must be evaluated carefully before it can be used in a robust way in hydromorphological interpretations.