The level of the River Lark is measured by the Environment Agency at several locations; the closest to us is at Temple Bridge near Icklingham. Data is available in close to real-time, both of the level (in m), and the flow (m3/sec). Historic data (going back many years) allows for comparison. 

River Lark at Temple

River level and wading 

Using the reported level at Temple as a surrogate for observed conditions on our own length of river, here is John Urquhart’s view, based on 0.15m intervals.

Over 0.75m: Flooding. Much too high to fish and definitely unsafe to enter the river. 
0.6m: Bank fishing possible. Wading unsafe. The Environment Agency considers levels over 0.62m at Temple as indicating risk of local flooding. 
0.45m: Wading possible with care. Cherry Tree Lock accessible - but only after first being guided through by an experienced member. 
0.3m: Typical summer level. Most of the river accessible in chest waders. 
0.15m: low water. Short rod, long leader, tiny flies. 
Below 0.15m: pour a beer and retreat under shade. 

John’s opinion is that chest waders are essential and that thigh waders invite complacency as there are unexpected depths and strong currents. One of the most important parts of wading kit is the belt, which traps air in the lower part of the waders and delivers buoyancy in case of immersion. Never enter the river without a stout wading staff and a whistle to attract attention in case of getting into difficulty. A flotation device - whether one that inflates on exposure to water, or a simpler life vest - is strongly recommended. 

When following a bank upstream, carry your wading staff on the outside, away from the bank, and, when crossing the river, carry it so that you are braced against the current, so on the downstream side. 

Silty parts of the river are in many ways as dangerous as any. Never enter any part of the river where you can't immediately plan a way out. 

Finally, although chest waders are recommended, it is risky to enter water higher than the bottom of your fishing vest - if that's getting wet (just above waist level) you’re in too deep. Retreat to safety is always available. There will be another opportunity.